.)
| or \|)
[ ... ] and [^ ... ])
( ... ) or \( ... \))
A regular expression (or regexp, or pattern) is a text string that describes some (mathematical) set of strings. A regexp r matches a string s if s is in the set of strings described by r.
Using the Regex library, you can:
Some regular expressions match only one string, i.e., the set they describe has only one member. For example, the regular expression `foo' matches the string `foo' and no others. Other regular expressions match more than one string, i.e., the set they describe has more than one member. For example, the regular expression `f*' matches the set of strings made up of any number (including zero) of `f's. As you can see, some characters in regular expressions match themselves (such as `f') and some don't (such as `*'); the ones that don't match themselves instead let you specify patterns that describe many different strings.
To either match or search for a regular expression with the Regex library functions, you must first compile it with a Regex pattern compiling function. A compiled pattern is a regular expression converted to the internal format used by the library functions. Once you've compiled a pattern, you can use it for matching or searching any number of times.
The Regex library consists of two source files: `regex.h' and `regex.c'. Regex provides three groups of functions with which you can operate on regular expressions. One group--the GNU group--is more powerful but not completely compatible with the other two, namely the POSIX and Berkeley UNIX groups; its interface was designed specifically for GNU. The other groups have the same interfaces as do the regular expression functions in POSIX and Berkeley UNIX.
We wrote this chapter with programmers in mind, not users of programs--such as Emacs--that use Regex. We describe the Regex library in its entirety, not how to write regular expressions that a particular program understands.
Characters are things you can type. Operators are things in a regular expression that match one or more characters. You compose regular expressions from operators, which in turn you specify using one or more characters.
Most characters represent what we call the match-self operator, i.e., they match themselves; we call these characters ordinary. Other characters represent either all or parts of fancier operators; e.g., `.' represents what we call the match-any-character operator (which, no surprise, matches (almost) any character); we call these characters special. Two different things determine what characters represent what operators:
In the following sections, we describe these things in more detail.
In any particular syntax for regular expressions, some characters are
always special, others are sometimes special, and others are never
special. The particular syntax that Regex recognizes for a given
regular expression depends on the value in the syntax field of
the pattern buffer of that regular expression.
You get a pattern buffer by compiling a regular expression. See section GNU Pattern Buffers, and section POSIX Pattern Buffers, for more information on pattern buffers. See section GNU Regular Expression Compiling, section POSIX Regular Expression Compiling, and section BSD Regular Expression Compiling, for more information on compiling.
Regex considers the value of the syntax field to be a collection
of bits; we refer to these bits as syntax bits. In most cases,
they affect what characters represent what operators. We describe the
meanings of the operators to which we refer in section Common Operators,
section GNU Operators, and section GNU Emacs Operators.
For reference, here is the complete list of syntax bits, in alphabetical order:
RE_BACKSLASH_ESCAPE_IN_LISTS
[ ... ] and [^ ... ])
quotes (makes ordinary, if it's special) the following character; if
this bit isn't set, then `\' is an ordinary character inside lists.
(See section The Backslash Character, for what `\' does outside of lists.)
RE_BK_PLUS_QM
RE_LIMITED_OPS is set.
RE_CHAR_CLASSES
RE_CONTEXT_INDEP_ANCHORS
^), and
section The Match-end-of-line Operator ($).
RE_CONTEXT_INDEP_OPS
RE_LIMITED_OPS
isn't set) `+' and `?' (or `\+' and `\?', depending
on the syntax bit RE_BK_PLUS_QM) represent repetition operators
only if they're not first in a regular expression or just after an
open-group or alternation operator. The same holds for `{' (or
`\{', depending on the syntax bit RE_NO_BK_BRACES) if
it is the beginning of a valid interval and the syntax bit
RE_INTERVALS is set.
RE_CONTEXT_INVALID_OPS
RE_DOT_NEWLINE
RE_DOT_NOT_NULL
RE_INTERVALS
RE_LIMITED_OPS
RE_NEWLINE_ALT
RE_NO_BK_BRACES
RE_INTERVALS is set.
RE_NO_BK_PARENS
RE_NO_BK_REFS
RE_NO_BK_VBAR
RE_LIMITED_OPS is set.
RE_NO_EMPTY_RANGES
RE_UNMATCHED_RIGHT_PAREN_ORD
RE_NO_BK_PARENS is set) to match `)'.
If you're programming with Regex, you can set a pattern buffer's
(see section GNU Pattern Buffers, and section POSIX Pattern Buffers)
syntax field either to an arbitrary combination of syntax bits
(see section Syntax Bits) or else to the configurations defined by Regex.
These configurations define the syntaxes used by certain
programs---GNU Emacs,
POSIX Awk,
traditional Awk,
Grep,
Egrep--in addition to syntaxes for POSIX basic and extended
regular expressions.
The predefined syntaxes--taken directly from `regex.h'---are:
#define RE_SYNTAX_EMACS 0 #define RE_SYNTAX_AWK \ (RE_BACKSLASH_ESCAPE_IN_LISTS | RE_DOT_NOT_NULL \ | RE_NO_BK_PARENS | RE_NO_BK_REFS \ | RE_NO_BK_VBAR | RE_NO_EMPTY_RANGES \ | RE_UNMATCHED_RIGHT_PAREN_ORD) #define RE_SYNTAX_POSIX_AWK \ (RE_SYNTAX_POSIX_EXTENDED | RE_BACKSLASH_ESCAPE_IN_LISTS) #define RE_SYNTAX_GREP \ (RE_BK_PLUS_QM | RE_CHAR_CLASSES \ | RE_HAT_LISTS_NOT_NEWLINE | RE_INTERVALS \ | RE_NEWLINE_ALT) #define RE_SYNTAX_EGREP \ (RE_CHAR_CLASSES | RE_CONTEXT_INDEP_ANCHORS \ | RE_CONTEXT_INDEP_OPS | RE_HAT_LISTS_NOT_NEWLINE \ | RE_NEWLINE_ALT | RE_NO_BK_PARENS \ | RE_NO_BK_VBAR) #define RE_SYNTAX_POSIX_EGREP \ (RE_SYNTAX_EGREP | RE_INTERVALS | RE_NO_BK_BRACES) /* P1003.2/D11.2, section 4.20.7.1, lines 5078ff. */ #define RE_SYNTAX_ED RE_SYNTAX_POSIX_BASIC #define RE_SYNTAX_SED RE_SYNTAX_POSIX_BASIC /* Syntax bits common to both basic and extended POSIX regex syntax. */ #define _RE_SYNTAX_POSIX_COMMON \ (RE_CHAR_CLASSES | RE_DOT_NEWLINE | RE_DOT_NOT_NULL \ | RE_INTERVALS | RE_NO_EMPTY_RANGES) #define RE_SYNTAX_POSIX_BASIC \ (_RE_SYNTAX_POSIX_COMMON | RE_BK_PLUS_QM) /* Differs from ..._POSIX_BASIC only in that RE_BK_PLUS_QM becomes RE_LIMITED_OPS, i.e., \? \+ \| are not recognized. Actually, this isn't minimal, since other operators, such as \`, aren't disabled. */ #define RE_SYNTAX_POSIX_MINIMAL_BASIC \ (_RE_SYNTAX_POSIX_COMMON | RE_LIMITED_OPS) #define RE_SYNTAX_POSIX_EXTENDED \ (_RE_SYNTAX_POSIX_COMMON | RE_CONTEXT_INDEP_ANCHORS \ | RE_CONTEXT_INDEP_OPS | RE_NO_BK_BRACES \ | RE_NO_BK_PARENS | RE_NO_BK_VBAR \ | RE_UNMATCHED_RIGHT_PAREN_ORD) /* Differs from ..._POSIX_EXTENDED in that RE_CONTEXT_INVALID_OPS replaces RE_CONTEXT_INDEP_OPS and RE_NO_BK_REFS is added. */ #define RE_SYNTAX_POSIX_MINIMAL_EXTENDED \ (_RE_SYNTAX_POSIX_COMMON | RE_CONTEXT_INDEP_ANCHORS \ | RE_CONTEXT_INVALID_OPS | RE_NO_BK_BRACES \ | RE_NO_BK_PARENS | RE_NO_BK_REFS \ | RE_NO_BK_VBAR | RE_UNMATCHED_RIGHT_PAREN_ORD)
POSIX generalizes the notion of a character to that of a collating element. It defines a collating element to be "a sequence of one or more bytes defined in the current collating sequence as a unit of collation."
This generalizes the notion of a character in two ways. First, a single character can map into two or more collating elements. For example, the German collates as the collating element `s' followed by another collating element `s'. Second, two or more characters can map into one collating element. For example, the Spanish `ll' collates after `l' and before `m'.
Since POSIX's "collating element" preserves the essential idea of a "character," we use the latter, more familiar, term in this document.
The `\' character has one of four different meanings, depending on the context in which you use it and what syntax bits are set (see section Syntax Bits). It can: 1) stand for itself, 2) quote the next character, 3) introduce an operator, or 4) do nothing.
[ ... ] and [^ ... ])) if the syntax bit
RE_BACKSLASH_ESCAPE_IN_LISTS is not set. For example, `[\]'
would match `\'.
RE_BACKSLASH_ESCAPE_IN_LISTS is set.
RE_BK_PLUS_QM, RE_NO_BK_BRACES, RE_NO_BK_VAR,
RE_NO_BK_PARENS, RE_NO_BK_REF in section Syntax Bits. Also:
\b)).
\B)).
\<)).
\>)).
\w)).
\W)).
emacs
defined, then `\sclass' represents the match-syntactic-class
operator and `\Sclass' represents the
match-not-syntactic-class operator (see section Syntactic Class Operators).
You compose regular expressions from operators. In the following sections, we describe the regular expression operators specified by POSIX; GNU also uses these. Most operators have more than one representation as characters. See section Regular Expression Syntax, for what characters represent what operators under what circumstances.
For most operators that can be represented in two ways, one
representation is a single character and the other is that character
preceded by `\'. For example, either `(' or `\('
represents the open-group operator. Which one does depends on the
setting of a syntax bit, in this case RE_NO_BK_PARENS. Why is
this so? Historical reasons dictate some of the varying
representations, while POSIX dictates others.
Finally, almost all characters lose any special meaning inside a list
(see section List Operators ([ ... ] and [^ ... ])).
This operator matches the character itself. All ordinary characters (see section Regular Expression Syntax) represent this operator. For example, `f' is always an ordinary character, so the regular expression `f' matches only the string `f'. In particular, it does not match the string `ff'.
.)This operator matches any single printing or nonprinting character except it won't match a:
RE_DOT_NEWLINE isn't set.
RE_DOT_NOT_NULL is set.
The `.' (period) character represents this operator. For example, `a.b' matches any three-character string beginning with `a' and ending with `b'.
This operator concatenates two regular expressions a and b. No character represents this operator; you simply put b after a. The result is a regular expression that will match a string if a matches its first part and b matches the rest. For example, `xy' (two match-self operators) matches `xy'.
Repetition operators repeat the preceding regular expression a specified number of times.
*)This operator repeats the smallest possible preceding regular expression as many times as necessary (including zero) to match the pattern. `*' represents this operator. For example, `o*' matches any string made up of zero or more `o's. Since this operator operates on the smallest preceding regular expression, `fo*' has a repeating `o', not a repeating `fo'. So, `fo*' matches `f', `fo', `foo', and so on.
Since the match-zero-or-more operator is a suffix operator, it may be useless as such when no regular expression precedes it. This is the case when it:
Three different things can happen in these cases:
RE_CONTEXT_INVALID_OPS is set, then the
regular expression is invalid.
RE_CONTEXT_INVALID_OPS isn't set, but
RE_CONTEXT_INDEP_OPS is, then `*' represents the
match-zero-or-more operator (which then operates on the empty string).
The matcher processes a match-zero-or-more operator by first matching as many repetitions of the smallest preceding regular expression as it can. Then it continues to match the rest of the pattern.
If it can't match the rest of the pattern, it backtracks (as many times as necessary), each time discarding one of the matches until it can either match the entire pattern or be certain that it cannot get a match. For example, when matching `ca*ar' against `caaar', the matcher first matches all three `a's of the string with the `a*' of the regular expression. However, it cannot then match the final `ar' of the regular expression against the final `r' of the string. So it backtracks, discarding the match of the last `a' in the string. It can then match the remaining `ar'.
+ or \+)
If the syntax bit RE_LIMITED_OPS is set, then Regex doesn't recognize
this operator. Otherwise, if the syntax bit RE_BK_PLUS_QM isn't
set, then `+' represents this operator; if it is, then `\+'
does.
This operator is similar to the match-zero-or-more operator except that
it repeats the preceding regular expression at least once;
see section The Match-zero-or-more Operator (*), for what it operates on, how some
syntax bits affect it, and how Regex backtracks to match it.
For example, supposing that `+' represents the match-one-or-more operator; then `ca+r' matches, e.g., `car' and `caaaar', but not `cr'.
? or \?)
If the syntax bit RE_LIMITED_OPS is set, then Regex doesn't
recognize this operator. Otherwise, if the syntax bit
RE_BK_PLUS_QM isn't set, then `?' represents this operator;
if it is, then `\?' does.
This operator is similar to the match-zero-or-more operator except that
it repeats the preceding regular expression once or not at all;
see section The Match-zero-or-more Operator (*), to see what it operates on, how
some syntax bits affect it, and how Regex backtracks to match it.
For example, supposing that `?' represents the match-zero-or-one operator; then `ca?r' matches both `car' and `cr', but nothing else.
{ ... } or \{ ... \})
If the syntax bit RE_INTERVALS is set, then Regex recognizes
interval expressions. They repeat the smallest possible preceding
regular expression a specified number of times.
If the syntax bit RE_NO_BK_BRACES is set, `{' represents
the open-interval operator and `}' represents the
close-interval operator ; otherwise, `\{' and `\}' do.
Specifically, supposing that `{' and `}' represent the open-interval and close-interval operators; then:
{count}
{min,}
{min, max}
The interval expression (but not necessarily the regular expression that contains it) is invalid if:
RE_DUP_MAX (which symbol `regex.h'
defines).
If the interval expression is invalid and the syntax bit
RE_NO_BK_BRACES is set, then Regex considers all the
characters in the would-be interval to be ordinary. If that bit
isn't set, then the regular expression is invalid.
If the interval expression is valid but there is no preceding regular
expression on which to operate, then if the syntax bit
RE_CONTEXT_INVALID_OPS is set, the regular expression is invalid.
If that bit isn't set, then Regex considers all the characters--other
than backslashes, which it ignores--in the would-be interval to be
ordinary.
| or \|)
If the syntax bit RE_LIMITED_OPS is set, then Regex doesn't
recognize this operator. Otherwise, if the syntax bit
RE_NO_BK_VBAR is set, then `|' represents this operator;
otherwise, `\|' does.
Alternatives match one of a choice of regular expressions: if you put the character(s) representing the alternation operator between any two regular expressions a and b, the result matches the union of the strings that a and b match. For example, supposing that `|' is the alternation operator, then `foo|bar|quux' would match any of `foo', `bar' or `quux'.
The alternation operator operates on the largest possible surrounding regular expressions. (Put another way, it has the lowest precedence of any regular expression operator.) Thus, the only way you can delimit its arguments is to use grouping. For example, if `(' and `)' are the open and close-group operators, then `fo(o|b)ar' would match either `fooar' or `fobar'. (`foo|bar' would match `foo' or `bar'.)
The matcher usually tries all combinations of alternatives so as to match the longest possible string. For example, when matching `(fooq|foo)*(qbarquux|bar)' against `fooqbarquux', it cannot take, say, the first ("depth-first") combination it could match, since then it would be content to match just `fooqbar'.
[ ... ] and [^ ... ])Lists, also called bracket expressions, are a set of one or more items. An item is a character, a character class expression, or a range expression. The syntax bits affect which kinds of items you can put in a list. We explain the last two items in subsections below. Empty lists are invalid.
A matching list matches a single character represented by one of the list items. You form a matching list by enclosing one or more items within an open-matching-list operator (represented by `[') and a close-list operator (represented by `]').
For example, `[ab]' matches either `a' or `b'. `[ad]*' matches the empty string and any string composed of just `a's and `d's in any order. Regex considers invalid a regular expression with a `[' but no matching `]'.
Nonmatching lists are similar to matching lists except that they match a single character not represented by one of the list items. You use an open-nonmatching-list operator (represented by `[^'(2)) instead of an open-matching-list operator to start a nonmatching list.
For example, `[^ab]' matches any character except `a' or `b'.
If the posix_newline field in the pattern buffer (see section GNU Pattern Buffers is set, then nonmatching lists do not match a newline.
Most characters lose any special meaning inside a list. The special characters inside a list follow.
RE_BACKSLASH_ESCAPE_IN_LISTS is
set.
[: ... :])) if the syntax bit RE_CHAR_CLASSES is set and what
follows is a valid character class expression.
RE_CHAR_CLASSES is set and what precedes it is an
open-character-class operator followed by a valid character class name.
-)) if it's
not first or last in a list or the ending point of a range.
All other characters are ordinary. For example, `[.*]' matches `.' and `*'.
[: ... :])
If the syntax bit RE_CHARACTER_CLASSES is set, then Regex
recognizes character class expressions inside lists. A character
class expression matches one character from a given class. You form a
character class expression by putting a character class name between an
open-character-class operator (represented by `[:') and a
close-character-class operator (represented by `:]'). The
character class names and their meanings are:
alnum
alpha
blank
cntrl
digit
graph
print except omits space
lower
print
punct
space
upper
xdigit
0--9, a--f, A--F
These correspond to the definitions in the C library's `<ctype.h>'
facility. For example, `[:alpha:]' corresponds to the standard
facility isalpha. Regex recognizes character class expressions
only inside of lists; so `[[:alpha:]]' matches any letter, but
`[:alpha:]' outside of a bracket expression and not followed by a
repetition operator matches just itself.
-)Regex recognizes range expressions inside a list. They represent those characters that fall between two elements in the current collating sequence. You form a range expression by putting a range operator between two characters.(3) `-' represents the range operator. For example, `a-f' within a list represents all the characters from `a' through `f' inclusively.
If the syntax bit RE_NO_EMPTY_RANGES is set, then if the range's
ending point collates less than its starting point, the range (and the
regular expression containing it) is invalid. For example, the regular
expression `[z-a]' would be invalid. If this bit isn't set, then
Regex considers such a range to be empty.
Since `-' represents the range operator, if you want to make a `-' character itself a list item, you must do one of the following:
For example, `[-a-z]' matches a lowercase letter or a hyphen (in English, in ASCII).
( ... ) or \( ... \))A group, also known as a subexpression, consists of an open-group operator, any number of other operators, and a close-group operator. Regex treats this sequence as a unit, just as mathematics and programming languages treat a parenthesized expression as a unit.
Therefore, using groups, you can:
| or \|)) or a repetition operator (see section Repetition Operators).
If the syntax bit RE_NO_BK_PARENS is set, then `(' represents
the open-group operator and `)' represents the
close-group operator; otherwise, `\(' and `\)' do.
If the syntax bit RE_UNMATCHED_RIGHT_PAREN_ORD is set and a
close-group operator has no matching open-group operator, then Regex
considers it to match `)'.
If the syntax bit RE_NO_BK_REF isn't set, then Regex recognizes
back references. A back reference matches a specified preceding group.
The back reference operator is represented by `\digit'
anywhere after the end of a regular expression's digit-th
group (see section Grouping Operators (( ... ) or \( ... \))).
digit must be between `1' and `9'. The matcher assigns numbers 1 through 9 to the first nine groups it encounters. By using one of `\1' through `\9' after the corresponding group's close-group operator, you can match a substring identical to the one that the group does.
Back references match according to the following (in all examples below, `(' represents the open-group, `)' the close-group, `{' the open-interval and `}' the close-interval operator):
RE_DOT_NEWLINE isn't set) string that is composed of two
identical halves; the `(.*)' matches the first half and the
`\1' matches the second half.
You can use a back reference as an argument to a repetition operator. For example, `(a(b))\2*' matches `a' followed by two or more `b's. Similarly, `(a(b))\2{3}' matches `abbbb'.
If there is no preceding digit-th subexpression, the regular expression is invalid.
These operators can constrain a pattern to match only at the beginning or end of the entire string or at the beginning or end of a line.
^)This operator can match the empty string either at the beginning of the string or after a newline character. Thus, it is said to anchor the pattern to the beginning of a line.
In the cases following, `^' represents this operator. (Otherwise, `^' is ordinary.)
RE_CONTEXT_INDEP_ANCHORS is set, and it is outside
a bracket expression.
( ... ) or \( ... \)), and section The Alternation Operator (| or \|).
These rules imply that some valid patterns containing `^' cannot be
matched; for example, `foo^bar' if RE_CONTEXT_INDEP_ANCHORS
is set.
If the not_bol field is set in the pattern buffer (see section GNU Pattern Buffers), then `^' fails to match at the beginning of the
string. See section POSIX Matching, for when you might find this useful.
If the newline_anchor field is set in the pattern buffer, then
`^' fails to match after a newline. This is useful when you do not
regard the string to be matched as broken into lines.
$)This operator can match the empty string either at the end of the string or before a newline character in the string. Thus, it is said to anchor the pattern to the end of a line.
It is always represented by `$'. For example, `foo$' usually matches, e.g., `foo' and, e.g., the first three characters of `foo\nbar'.
Its interaction with the syntax bits and pattern buffer fields is exactly the dual of `^''s; see the previous section. (That is, "beginning" becomes "end", "next" becomes "previous", and "after" becomes "before".)
Following are operators that GNU defines (and POSIX doesn't).
The operators in this section require Regex to recognize parts of words. Regex uses a syntax table to determine whether or not a character is part of a word, i.e., whether or not it is word-constituent.
A syntax table is an array indexed by the characters in your
character set. In the ASCII encoding, therefore, a syntax table
has 256 elements. Regex always uses a char * variable
re_syntax_table as its syntax table. In some cases, it
initializes this variable and in others it expects you to initialize it.
emacs and
SYNTAX_TABLE both undefined, then Regex allocates
re_syntax_table and initializes an element i either to
Sword (which it defines) if i is a letter, number, or
`_', or to zero if it's not.
emacs undefined but SYNTAX_TABLE
defined, then Regex expects you to define a char * variable
re_syntax_table to be a valid syntax table.
emacs defined.
\b)This operator (represented by `\b') matches the empty string at either the beginning or the end of a word. For example, `\brat\b' matches the separate word `rat'.
\B)This operator (represented by `\B') matches the empty string within a word. For example, `c\Brat\Be' matches `crate', but `dirty \Brat' doesn't match `dirty rat'.
\<)This operator (represented by `\<') matches the empty string at the beginning of a word.
\>)This operator (represented by `\>') matches the empty string at the end of a word.
\w)This operator (represented by `\w') matches any word-constituent character.
\W)This operator (represented by `\W') matches any character that is not word-constituent.
Following are operators which work on buffers. In Emacs, a buffer is, naturally, an Emacs buffer. For other programs, Regex considers the entire string to be matched as the buffer.
\`)This operator (represented by `\`') matches the empty string at the beginning of the buffer.
\')This operator (represented by `\'') matches the empty string at the end of the buffer.
Following are operators that GNU defines (and POSIX doesn't)
that you can use only when Regex is compiled with the preprocessor
symbol emacs defined.
The operators in this section require Regex to recognize the syntactic classes of characters. Regex uses a syntax table to determine this.
A syntax table is an array indexed by the characters in your character set. In the ASCII encoding, therefore, a syntax table has 256 elements.
If Regex is compiled with the preprocessor symbol emacs defined,
then Regex expects you to define and initialize the variable
re_syntax_table to be an Emacs syntax table. Emacs' syntax
tables are more complicated than Regex's own (see section Non-Emacs Syntax Tables). See section `Syntax' in The GNU Emacs User's Manual,
for a description of Emacs' syntax tables.
\sclass)This operator matches any character whose syntactic class is represented by a specified character. `\sclass' represents this operator where class is the character representing the syntactic class you want. For example, `w' represents the syntactic class of word-constituent characters, so `\sw' matches any word-constituent character.
\Sclass)This operator is similar to the match-syntactic-class operator except that it matches any character whose syntactic class is not represented by the specified character. `\Sclass' represents this operator. For example, `w' represents the syntactic class of word-constituent characters, so `\Sw' matches any character that is not word-constituent.
Regex usually matches strings according to the "leftmost longest" rule; that is, it chooses the longest of the leftmost matches. This does not mean that for a regular expression containing subexpressions that it simply chooses the longest match for each subexpression, left to right; the overall match must also be the longest possible one.
For example, `(ac*)(c*d[ac]*)\1' matches `acdacaaa', not `acdac', as it would if it were to choose the longest match for the first subexpression.
Here we describe how you use the Regex data structures and functions in C programs. Regex has three interfaces: one designed for GNU, one compatible with POSIX and one compatible with Berkeley UNIX.
If you're writing code that doesn't need to be compatible with either POSIX or Berkeley UNIX, you can use these functions. They provide more options than the other interfaces.
To compile, match, or search for a given regular expression, you must supply a pattern buffer. A pattern buffer holds one compiled regular expression.(4)
You can have several different pattern buffers simultaneously, each holding a compiled pattern for a different regular expression.
`regex.h' defines the pattern buffer struct as follows:
/* Space that holds the compiled pattern. It is declared as
`unsigned char *' because its elements are
sometimes used as array indexes. */
unsigned char *buffer;
/* Number of bytes to which `buffer' points. */
unsigned long allocated;
/* Number of bytes actually used in `buffer'. */
unsigned long used;
/* Syntax setting with which the pattern was compiled. */
reg_syntax_t syntax;
/* Pointer to a fastmap, if any, otherwise zero. re_search uses
the fastmap, if there is one, to skip over impossible
starting points for matches. */
char *fastmap;
/* Either a translate table to apply to all characters before
comparing them, or zero for no translation. The translation
is applied to a pattern when it is compiled and to a string
when it is matched. */
char *translate;
/* Number of subexpressions found by the compiler. */
size_t re_nsub;
/* Zero if this pattern cannot match the empty string, one else.
Well, in truth it's used only in `re_search_2', to see
whether or not we should use the fastmap, so we don't set
this absolutely perfectly; see `re_compile_fastmap' (the
`duplicate' case). */
unsigned can_be_null : 1;
/* If REGS_UNALLOCATED, allocate space in the `regs' structure
for `max (RE_NREGS, re_nsub + 1)' groups.
If REGS_REALLOCATE, reallocate space if necessary.
If REGS_FIXED, use what's there. */
#define REGS_UNALLOCATED 0
#define REGS_REALLOCATE 1
#define REGS_FIXED 2
unsigned regs_allocated : 2;
/* Set to zero when `regex_compile' compiles a pattern; set to one
by `re_compile_fastmap' if it updates the fastmap. */
unsigned fastmap_accurate : 1;
/* If set, `re_match_2' does not return information about
subexpressions. */
unsigned no_sub : 1;
/* If set, a beginning-of-line anchor doesn't match at the
beginning of the string. */
unsigned not_bol : 1;
/* Similarly for an end-of-line anchor. */
unsigned not_eol : 1;
/* If true, an anchor at a newline matches. */
unsigned newline_anchor : 1;
In GNU, you can both match and search for a given regular expression. To do either, you must first compile it in a pattern buffer (see section GNU Pattern Buffers).
Regular expressions match according to the syntax with which they were
compiled; with GNU, you indicate what syntax you want by setting
the variable re_syntax_options (declared in `regex.h' and
defined in `regex.c') before calling the compiling function,
re_compile_pattern (see below). See section Syntax Bits, and
section Predefined Syntaxes.
You can change the value of re_syntax_options at any time.
Usually, however, you set its value once and then never change it.
re_compile_pattern takes a pattern buffer as an argument. You
must initialize the following fields:
translate initialization
translate
fastmap
buffer
allocated
re_compile_pattern to allocate memory for the
compiled pattern, set both of these to zero. If you have an existing
block of memory (allocated with malloc) you want Regex to use,
set buffer to its address and allocated to its size (in
bytes).
re_compile_pattern uses realloc to extend the space for
the compiled pattern as necessary.
To compile a pattern buffer, use:
char *
re_compile_pattern (const char *regex, const int regex_size,
struct re_pattern_buffer *pattern_buffer)
regex is the regular expression's address, regex_size is its length, and pattern_buffer is the pattern buffer's address.
If re_compile_pattern successfully compiles the regular
expression, it returns zero and sets *pattern_buffer to the
compiled pattern. It sets the pattern buffer's fields as follows:
buffer
used
buffer occupies.
syntax
re_syntax_options.
re_nsub
fastmap_accurate
buffer; in that case (since
you can't make a fastmap without a compiled pattern),
fastmap would either contain an incompatible fastmap, or nothing
at all.
If re_compile_pattern can't compile regex, it returns an
error string corresponding to one of the errors listed in section POSIX Regular Expression Compiling.
Matching the GNU way means trying to match as much of a string as possible starting at a position within it you specify. Once you've compiled a pattern into a pattern buffer (see section GNU Regular Expression Compiling), you can ask the matcher to match that pattern against a string using:
int
re_match (struct re_pattern_buffer *pattern_buffer,
const char *string, const int size,
const int start, struct re_registers *regs)
pattern_buffer is the address of a pattern buffer containing a compiled pattern. string is the string you want to match; it can contain newline and null characters. size is the length of that string. start is the string index at which you want to begin matching; the first character of string is at index zero. See section Using Registers, for a explanation of regs; you can safely pass zero.
re_match matches the regular expression in pattern_buffer
against the string string according to the syntax in
pattern_buffers's syntax field. (See section GNU Regular Expression Compiling, for how to set it.) The function returns
-1 if the compiled pattern does not match any part of
string and -2 if an internal error happens; otherwise, it
returns how many (possibly zero) characters of string the pattern
matched.
An example: suppose pattern_buffer points to a pattern buffer
containing the compiled pattern for `a*', and string points
to `aaaaab' (whereupon size should be 6). Then if start
is 2, re_match returns 3, i.e., `a*' would have matched the
last three `a's in string. If start is 0,
re_match returns 5, i.e., `a*' would have matched all the
`a's in string. If start is either 5 or 6, it returns
zero.
If start is not between zero and size, then
re_match returns -1.
Searching means trying to match starting at successive positions
within a string. The function re_search does this.
Before calling re_search, you must compile your regular
expression. See section GNU Regular Expression Compiling.
Here is the function declaration:
int
re_search (struct re_pattern_buffer *pattern_buffer,
const char *string, const int size,
const int start, const int range,
struct re_registers *regs)
whose arguments are the same as those to re_match (see section GNU Matching) except that the two arguments start and range
replace re_match's argument start.
If range is positive, then re_search attempts a match
starting first at index start, then at start + 1 if
that fails, and so on, up to start + range; if
range is negative, then it attempts a match starting first at
index start, then at start -1 if that fails, and so
on.
If start is not between zero and size, then re_search
returns -1. When range is positive, re_search
adjusts range so that start + range - 1 is
between zero and size, if necessary; that way it won't search
outside of string. Similarly, when range is negative,
re_search adjusts range so that start +
range + 1 is between zero and size, if necessary.
If the fastmap field of pattern_buffer is zero,
re_search matches starting at consecutive positions; otherwise,
it uses fastmap to make the search more efficient.
See section Searching with Fastmaps.
If no match is found, re_search returns -1. If
a match is found, it returns the index where the match began. If an
internal error happens, it returns -2.
Using the functions re_match_2 and re_search_2, you can
match or search in data that is divided into two strings.
The function:
int
re_match_2 (struct re_pattern_buffer *buffer,
const char *string1, const int size1,
const char *string2, const int size2,
const int start,
struct re_registers *regs,
const int stop)
is similar to re_match (see section GNU Matching) except that you
pass two data strings and sizes, and an index stop beyond
which you don't want the matcher to try matching. As with
re_match, if it succeeds, re_match_2 returns how many
characters of string it matched. Regard string1 and
string2 as concatenated when you set the arguments start and
stop and use the contents of regs; re_match_2 never
returns a value larger than size1 + size2.
The function:
int
re_search_2 (struct re_pattern_buffer *buffer,
const char *string1, const int size1,
const char *string2, const int size2,
const int start, const int range,
struct re_registers *regs,
const int stop)
is similarly related to re_search.
If you're searching through a long string, you should use a fastmap. Without one, the searcher tries to match at consecutive positions in the string. Generally, most of the characters in the string could not start a match. It takes much longer to try matching at a given position in the string than it does to check in a table whether or not the character at that position could start a match. A fastmap is such a table.
More specifically, a fastmap is an array indexed by the characters in
your character set. Under the ASCII encoding, therefore, a fastmap
has 256 elements. If you want the searcher to use a fastmap with a
given pattern buffer, you must allocate the array and assign the array's
address to the pattern buffer's fastmap field. You either can
compile the fastmap yourself or have re_search do it for you;
when fastmap is nonzero, it automatically compiles a fastmap the
first time you search using a particular compiled pattern.
To compile a fastmap yourself, use:
int re_compile_fastmap (struct re_pattern_buffer *pattern_buffer)
pattern_buffer is the address of a pattern buffer. If the
character c could start a match for the pattern,
re_compile_fastmap makes
pattern_buffer->fastmap[c] nonzero. It returns
0 if it can compile a fastmap and -2 if there is an
internal error. For example, if `|' is the alternation operator
and pattern_buffer holds the compiled pattern for `a|b', then
re_compile_fastmap sets fastmap['a'] and
fastmap['b'] (and no others).
re_search uses a fastmap as it moves along in the string: it
checks the string's characters until it finds one that's in the fastmap.
Then it tries matching at that character. If the match fails, it
repeats the process. So, by using a fastmap, re_search doesn't
waste time trying to match at positions in the string that couldn't
start a match.
If you don't want re_search to use a fastmap,
store zero in the fastmap field of the pattern buffer before
calling re_search.
Once you've initialized a pattern buffer's fastmap field, you
need never do so again--even if you compile a new pattern in
it--provided the way the field is set still reflects whether or not you
want a fastmap. re_search will still either do nothing if
fastmap is null or, if it isn't, compile a new fastmap for the
new pattern.
If you set the translate field of a pattern buffer to a translate
table, then the GNU Regex functions to which you've passed that
pattern buffer use it to apply a simple transformation
to all the regular expression and string characters at which they look.
A translate table is an array indexed by the characters in your
character set. Under the ASCII encoding, therefore, a translate
table has 256 elements. The array's elements are also characters in
your character set. When the Regex functions see a character c,
they use translate[c] in its place, with one exception: the
character after a `\' is not translated. (This ensures that, the
operators, e.g., `\B' and `\b', are always distinguishable.)
For example, a table that maps all lowercase letters to the
corresponding uppercase ones would cause the matcher to ignore
differences in case.(5) Such a table would map all characters except lowercase letters
to themselves, and lowercase letters to the corresponding uppercase
ones. Under the ASCII encoding, here's how you could initialize
such a table (we'll call it case_fold):
for (i = 0; i < 256; i++)
case_fold[i] = i;
for (i = 'a'; i <= 'z'; i++)
case_fold[i] = i - ('a' - 'A');
You tell Regex to use a translate table on a given pattern buffer by
assigning that table's address to the translate field of that
buffer. If you don't want Regex to do any translation, put zero into
this field. You'll get weird results if you change the table's contents
anytime between compiling the pattern buffer, compiling its fastmap, and
matching or searching with the pattern buffer.
A group in a regular expression can match a (posssibly empty) substring of the string that regular expression as a whole matched. The matcher remembers the beginning and end of the substring matched by each group.
To find out what they matched, pass a nonzero regs argument to a GNU matching or searching function (see section GNU Matching and section GNU Searching), i.e., the address of a structure of this type, as defined in `regex.h':
struct re_registers
{
unsigned num_regs;
regoff_t *start;
regoff_t *end;
};
Except for (possibly) the num_regs'th element (see below), the
ith element of the start and end arrays records
information about the ith group in the pattern. (They're declared
as C pointers, but this is only because not all C compilers accept
zero-length arrays; conceptually, it is simplest to think of them as
arrays.)
The start and end arrays are allocated in various ways,
depending on the value of the regs_allocated
field in the pattern buffer passed to the matcher.
The simplest and perhaps most useful is to let the matcher (re)allocate
enough space to record information for all the groups in the regular
expression. If regs_allocated is REGS_UNALLOCATED,
the matcher allocates 1 + re_nsub (another field in the
pattern buffer; see section GNU Pattern Buffers). The extra element is set
to -1, and sets regs_allocated to REGS_REALLOCATE.
Then on subsequent calls with the same pattern buffer and regs
arguments, the matcher reallocates more space if necessary.
It would perhaps be more logical to make the regs_allocated field
part of the re_registers structure, instead of part of the
pattern buffer. But in that case the caller would be forced to
initialize the structure before passing it. Much existing code doesn't
do this initialization, and it's arguably better to avoid it anyway.
re_compile_pattern sets regs_allocated to
REGS_UNALLOCATED,
so if you use the GNU regular expression
functions, you get this behavior by default.
xx document re_set_registers
POSIX, on the other hand, requires a different interface: the
caller is supposed to pass in a fixed-length array which the matcher
fills. Therefore, if regs_allocated is REGS_FIXED
the matcher simply fills that array.
The following examples illustrate the information recorded in the
re_registers structure. (In all of them, `(' represents the
open-group and `)' the close-group operator. The first character
in the string string is at index 0.)
regs->start[i] to the index in string where
the substring matched by the i-th group begins, and
regs->end[i] to the index just beyond that
substring's end. The function sets regs->start[0] and
regs->end[0] to analogous information about the entire
pattern.
For example, when you match `((a)(b))' against `ab', you get:
regs->start[0] and 2 in regs->end[0]
regs->start[1] and 2 in regs->end[1]
regs->start[2] and 1 in regs->end[2]
regs->start[3] and 2 in regs->end[3]
regs->start[0] and 2 in regs->end[0]
regs->start[1] and 2 in regs->end[1]
regs->start[i] and
regs->end[i] to -1.
For example, when you match the pattern `(a)*b' against
the string `b', you get:
regs->start[0] and 1 in regs->end[0]
regs->start[1] and -1 in regs->end[1]
regs->start[i] and
regs->end[i] to the index just beyond that
zero-length string.
For example, when you match the pattern `(a*)b' against the string
`b', you get:
regs->start[0] and 1 in regs->end[0]
regs->start[1] and 0 in regs->end[1]
regs->start[j] and
regs->end[j] the last match (if it matched) of
the j-th group.
For example, when you match the pattern `((a*)b)*' against the
string `abb', group 2 last matches the empty string, so you
get what it previously matched:
regs->start[0] and 3 in regs->end[0]
regs->start[1] and 3 in regs->end[1]
regs->start[2] and 2 in regs->end[2]
regs->start[0] and 3 in regs->end[0]
regs->start[1] and 3 in regs->end[1]
regs->start[2] and 1 in regs->end[2]
regs->start[i] and
regs->end[i] to -1, then it also sets
regs->start[j] and
regs->end[j] to -1.
For example, when you match the pattern `((a)*b)*c' against the
string `c', you get:
regs->start[0] and 1 in regs->end[0]
regs->start[1] and -1 in regs->end[1]
regs->start[2] and -1 in regs->end[2]
To free any allocated fields of a pattern buffer, you can use the
POSIX function described in section Freeing POSIX Pattern Buffers,
since the type regex_t---the type for POSIX pattern
buffers--is equivalent to the type re_pattern_buffer. After
freeing a pattern buffer, you need to again compile a regular expression
in it (see section GNU Regular Expression Compiling) before passing it to
a matching or searching function.
If you're writing code that has to be POSIX compatible, you'll need to use these functions. Their interfaces are as specified by POSIX, draft 1003.2/D11.2.
To compile or match a given regular expression the POSIX way, you
must supply a pattern buffer exactly the way you do for GNU
(see section GNU Pattern Buffers). POSIX pattern buffers have type
regex_t, which is equivalent to the GNU pattern buffer
type re_pattern_buffer.
With POSIX, you can only search for a given regular expression; you
can't match it. To do this, you must first compile it in a
pattern buffer, using regcomp.
To compile a pattern buffer, use:
int regcomp (regex_t *preg, const char *regex, int cflags)
preg is the initialized pattern buffer's address, regex is the regular expression's address, and cflags is the compilation flags, which Regex considers as a collection of bits. Here are the valid bits, as defined in `regex.h':
REG_EXTENDED
regcomp sets preg's syntax field accordingly.
REG_ICASE
regcomp sets preg's translate
field to a translate table which ignores case, replacing anything you've
put there before.
REG_NOSUB
no_sub field; see section POSIX Matching,
for what this means.
REG_NEWLINE
.)) doesn't match a newline.
[ ... ] and [^ ... ])) matches a newline.
^)) matches the empty string immediately after a newline,
regardless of how REG_NOTBOL is set (see section POSIX Matching, for
an explanation of REG_NOTBOL).
^)) matches the empty string immediately before a newline,
regardless of how REG_NOTEOL is set (see section POSIX Matching,
for an explanation of REG_NOTEOL).
If regcomp successfully compiles the regular expression, it
returns zero and sets *pattern_buffer to the compiled
pattern. Except for syntax (which it sets as explained above), it
also sets the same fields the same way as does the GNU compiling
function (see section GNU Regular Expression Compiling).
If regcomp can't compile the regular expression, it returns one
of the error codes listed here. (Except when noted differently, the
syntax of in all examples below is basic regular expression syntax.)
REG_BADRPT
REG_BADBR
REG_EBRACE
REG_EBRACK
REG_ERANGE
REG_ECTYPE
REG_EPAREN
REG_ESUBREG
REG_EEND
REG_EESCAPE
REG_BADPAT
REG_ESIZE
REG_ESPACE
Matching the POSIX way means trying to match a null-terminated string starting at its first character. Once you've compiled a pattern into a pattern buffer (see section POSIX Regular Expression Compiling), you can ask the matcher to match that pattern against a string using:
int
regexec (const regex_t *preg, const char *string,
size_t nmatch, regmatch_t pmatch[], int eflags)
preg is the address of a pattern buffer for a compiled pattern. string is the string you want to match.
See section Using Byte Offsets, for an explanation of pmatch. If you
pass zero for nmatch or you compiled preg with the
compilation flag REG_NOSUB set, then regexec will ignore
pmatch; otherwise, you must allocate it to have at least
nmatch elements. regexec will record nmatch byte
offsets in pmatch, and set to -1 any unused elements up to
pmatch[nmatch] - 1.
eflags specifies execution flags---namely, the two bits
REG_NOTBOL and REG_NOTEOL (defined in `regex.h'). If
you set REG_NOTBOL, then the match-beginning-of-line operator
(see section The Match-beginning-of-line Operator (^)) always fails to match.
This lets you match against pieces of a line, as you would need to if,
say, searching for repeated instances of a given pattern in a line; it
would work correctly for patterns both with and without
match-beginning-of-line operators. REG_NOTEOL works analogously
for the match-end-of-line operator (see section The Match-end-of-line Operator ($)); it exists for symmetry.
regexec tries to find a match for preg in string
according to the syntax in preg's syntax field.
(See section POSIX Regular Expression Compiling, for how to set it.) The
function returns zero if the compiled pattern matches string and
REG_NOMATCH (defined in `regex.h') if it doesn't.
If either regcomp or regexec fail, they return a nonzero
error code, the possibilities for which are defined in `regex.h'.
See section POSIX Regular Expression Compiling, and section POSIX Matching, for
what these codes mean. To get an error string corresponding to these
codes, you can use:
size_t
regerror (int errcode,
const regex_t *preg,
char *errbuf,
size_t errbuf_size)
errcode is an error code, preg is the address of the pattern buffer which provoked the error, errbuf is the error buffer, and errbuf_size is errbuf's size.
regerror returns the size in bytes of the error string
corresponding to errcode (including its terminating null). If
errbuf and errbuf_size are nonzero, it also returns in
errbuf the first errbuf_size - 1 characters of the
error string, followed by a null.
errbuf_size must be a nonnegative number less than or equal to the
size in bytes of errbuf.
You can call regerror with a null errbuf and a zero
errbuf_size to determine how large errbuf need be to
accommodate regerror's error string.
In POSIX, variables of type regmatch_t hold analogous
information, but are not identical to, GNU's registers (see section Using Registers). To get information about registers in POSIX, pass to
regexec a nonzero pmatch of type regmatch_t, i.e.,
the address of a structure of this type, defined in
`regex.h':
typedef struct
{
regoff_t rm_so;
regoff_t rm_eo;
} regmatch_t;
When reading in section Using Registers, about how the matching function
stores the information into the registers, substitute pmatch for
regs, pmatch[i]->rm_so for
regs->start[i] and
pmatch[i]->rm_eo for
regs->end[i].
To free any allocated fields of a pattern buffer, use:
void regfree (regex_t *preg)
preg is the pattern buffer whose allocated fields you want freed.
regfree also sets preg's allocated and used
fields to zero. After freeing a pattern buffer, you need to again
compile a regular expression in it (see section POSIX Regular Expression Compiling) before passing it to the matching function (see section POSIX Matching).
If you're writing code that has to be Berkeley UNIX compatible, you'll need to use these functions whose interfaces are the same as those in Berkeley UNIX.
With Berkeley UNIX, you can only search for a given regular
expression; you can't match one. To search for it, you must first
compile it. Before you compile it, you must indicate the regular
expression syntax you want it compiled according to by setting the
variable re_syntax_options (declared in `regex.h' to some
syntax (see section Regular Expression Syntax).
To compile a regular expression use:
char * re_comp (char *regex)
regex is the address of a null-terminated regular expression.
re_comp uses an internal pattern buffer, so you can use only the
most recently compiled pattern buffer. This means that if you want to
use a given regular expression that you've already compiled--but it
isn't the latest one you've compiled--you'll have to recompile it. If
you call re_comp with the null string (not the empty
string) as the argument, it doesn't change the contents of the pattern
buffer.
If re_comp successfully compiles the regular expression, it
returns zero. If it can't compile the regular expression, it returns
an error string. re_comp's error messages are identical to those
of re_compile_pattern (see section GNU Regular Expression Compiling).
Searching the Berkeley UNIX way means searching in a string
starting at its first character and trying successive positions within
it to find a match. Once you've compiled a pattern using re_comp
(see section BSD Regular Expression Compiling), you can ask Regex
to search for that pattern in a string using:
int re_exec (char *string)
string is the address of the null-terminated string in which you want to search.
re_exec returns either 1 for success or 0 for failure. It
automatically uses a GNU fastmap (see section Searching with Fastmaps).
Version 2, June 1991
Copyright © 1989, 1991 Free Software Foundation, Inc. 675 Mass Ave, Cambridge, MA 02139, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.
NO WARRANTY
If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does. Copyright (C) 19yy name of author This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) 19yy name of author Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details.
The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items--whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:
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re_compile_pattern
struct re_registers
re_compile_pattern
struct re_registers
re_search
re_compile_pattern
re_search
struct re_registers
re_compile_pattern
re_compile_pattern
Sometimes
you don't have to explicitly quote special characters to make
them ordinary. For instance, most characters lose any special meaning
inside a list (see section List Operators ([ ... ] and [^ ... ])). In addition, if the syntax bits
RE_CONTEXT_INVALID_OPS and RE_CONTEXT_INDEP_OPS
aren't set, then (for historical reasons) the matcher considers special
characters ordinary if they are in contexts where the operations they
represent make no sense; for example, then the match-zero-or-more
operator (represented by `*') matches itself in the regular
expression `*foo' because there is no preceding expression on which
it can operate. It is poor practice, however, to depend on this
behavior; if you want a special character to be ordinary outside a list,
it's better to always quote it, regardless.
Regex therefore doesn't consider the `^' to be the first character in the list. If you put a `^' character first in (what you think is) a matching list, you'll turn it into a nonmatching list.
You can't use a character class for the starting or ending point of a range, since a character class is not a single character.
Regular expressions are also referred to as "patterns," hence the name "pattern buffer."
A table that maps all uppercase letters to the corresponding lowercase ones would work just as well for this purpose.
This document was generated on 14 May 1998 using the texi2html translator version 1.51a.