This manual documents how to program with the GNU regular expression library. This is edition 0.12a of the manual, 19 September 1992.
The first part of this master menu lists the major nodes in this Info document, including the index. The rest of the menu lists all the lower level nodes in the document.
Regular Expression Syntax
Common Operators
Repetition Operators
List Operators ([ ... ] and [^ ... ])
Anchoring Operators
GNU Operators
Word Operators
Buffer Operators
GNU Emacs Operators
Syntactic Class Operators
Programming with Regex
GNU Regex Functions
POSIX Regex Functions
BSD Regex Functions
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 fs. 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 GNU Pattern Buffers, and POSIX Pattern Buffers, for more information on pattern buffers. See GNU Regular Expression Compiling, POSIX Regular Expression Compiling, and 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 Common Operators,
GNU Operators, and GNU Emacs Operators.
For reference, here is the complete list of syntax bits, in alphabetical order:
RE_BACKSLASH_ESCAPE_IN_LISTSRE_BK_PLUS_QMRE_LIMITED_OPS is set.
RE_CHAR_CLASSESRE_CONTEXT_INDEP_ANCHORSRE_CONTEXT_INDEP_OPSRE_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_OPSIf this bit isn't set, then you can put the characters representing the repetition and alternation characters anywhere in a regular expression. Whether or not they will in fact be operators in certain positions depends on other syntax bits.
RE_DOT_NEWLINERE_DOT_NOT_NULLRE_INTERVALSRE_LIMITED_OPSRE_NEWLINE_ALTRE_NO_BK_BRACESRE_INTERVALS is set.
RE_NO_BK_PARENSRE_NO_BK_REFSRE_NO_BK_VBARRE_LIMITED_OPS is set.
RE_NO_EMPTY_RANGESRE_UNMATCHED_RIGHT_PAREN_ORDRE_NO_BK_PARENS is set) to match ).
If you're programming with Regex, you can set a pattern buffer's,
(see GNU Pattern Buffers, and POSIX Pattern Buffers,
syntax field either to an arbitrary combination of syntax bits
(see 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_DOT_NEWLINE | RE_CONTEXT_INDEP_ANCHORS \
| RE_UNMATCHED_RIGHT_PAREN_ORD | RE_NO_GNU_OPS)
#define RE_SYNTAX_GNU_AWK \
((RE_SYNTAX_POSIX_EXTENDED | RE_BACKSLASH_ESCAPE_IN_LISTS | RE_DEBUG) \
& ~(RE_DOT_NOT_NULL | RE_INTERVALS | RE_CONTEXT_INDEP_OPS \
| RE_CONTEXT_INVALID_OPS ))
#define RE_SYNTAX_POSIX_AWK \
(RE_SYNTAX_POSIX_EXTENDED | RE_BACKSLASH_ESCAPE_IN_LISTS \
| RE_INTERVALS | RE_NO_GNU_OPS)
#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 \
| RE_INVALID_INTERVAL_ORD)
/* 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 | RE_CONTEXT_INVALID_DUP)
/* 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_CONTEXT_INVALID_OPS | RE_UNMATCHED_RIGHT_PAREN_ORD)
/* Differs from ..._POSIX_EXTENDED in that RE_CONTEXT_INDEP_OPS is
removed 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 Syntax Bits). It can: 1) stand for itself, 2) quote the next character, 3) introduce an operator, or 4) do nothing.
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 Syntax Bits. Also:
emacs
defined, then \sclass represents the match-syntactic-class
operator and \Sclass represents the
match-not-syntactic-class operator (see 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 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 List Operators).
This operator matches the character itself. All ordinary characters (see 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 os. 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 as 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 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 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 as and ds 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 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.
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.
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:
alnumalphablankcntrldigitgraphprint except omits space
lowerprintpunctspaceupperxdigit0–9, 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:
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 Grouping Operators).
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 bs. 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.
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 GNU Pattern Buffers), then ^ fails to match at the beginning of the
string. See 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 Non-Emacs Syntax Tables). See Syntax (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:
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 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 Syntax Bits, and
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
translatefastmapbufferallocatedre_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:
bufferusedbuffer occupies.
syntaxre_syntax_options.
re_nsubfastmap_accuratebuffer; 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 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 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 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 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 as in string. If start is 0,
re_match returns 5, i.e., a* would have matched all the
as 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 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 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 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 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 GNU Matching and 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 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.)
->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:
->start[0] and 2 in regs->end[0]
->start[1] and 2 in regs->end[1]
->start[2] and 1 in regs->end[2]
->start[3] and 2 in regs->end[3]
For example, when you match the pattern (a)* against the string aa, you get:
->start[0] and 2 in regs->end[0]
->start[1] and 2 in regs->end[1]
->start[i] and
regs->end[i] to -1.
For example, when you match the pattern (a)*b against the string b, you get:
->start[0] and 1 in regs->end[0]
->start[1] and -1 in regs->end[1]
->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:
->start[0] and 1 in regs->end[0]
->start[1] and 0 in regs->end[1]
->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:
->start[0] and 3 in regs->end[0]
->start[1] and 3 in regs->end[1]
->start[2] and 2 in regs->end[2]
When you match the pattern ((a)*b)* against the string abb, group 2 doesn't participate in the last match, so you get:
->start[0] and 3 in regs->end[0]
->start[1] and 3 in regs->end[1]
->start[2] and 1 in regs->end[2]
->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:
->start[0] and 1 in regs->end[0]
->start[1] and -1 in regs->end[1]
->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 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 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 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_EXTENDEDregcomp sets preg's syntax field accordingly.
REG_ICASEregcomp sets preg's translate
field to a translate table which ignores case, replacing anything you've
put there before.
REG_NOSUBno_sub field; see POSIX Matching,
for what this means.
REG_NEWLINEREG_NOTBOL is set (see POSIX Matching, for
an explanation of REG_NOTBOL).
REG_NOTEOL is set (see 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 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_BADRPTREG_BADBRREG_EBRACEREG_EBRACKREG_ERANGEREG_ECTYPEREG_EPARENREG_ESUBREGREG_EENDREG_EESCAPEREG_BADPATREG_ESIZEREG_ESPACEMatching 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 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 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 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 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 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 POSIX Regular Expression Compiling, and 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 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 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 POSIX Regular Expression Compiling) before passing it to the matching function (see 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 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 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 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 Searching with Fastmaps).
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.
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one line to give the program's name and a brief idea of what it does.
Copyright (C) 19yy name of author
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it under the terms of the GNU General Public License as published by
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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.
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`Gnomovision' (which makes passes at compilers) written by James Hacker.
signature of Ty Coon, 1 April 1989
Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
$: Match-end-of-line Operator(: Grouping Operators): Grouping Operators\(: Grouping Operators\): Grouping Operators\|: Alternation Operator^: Match-beginning-of-line Operatorallocated initialization: GNU Regular Expression Compilingbuffer field, set by re_compile_pattern: GNU Regular Expression Compilingbuffer initialization: GNU Regular Expression Compilingend in struct re_registers: Using Registersfastmap initialization: GNU Regular Expression Compilingfastmap_accurate field, set by re_compile_pattern: GNU Regular Expression Compilingnewline_anchor field in pattern buffer: Match-beginning-of-line Operatornot_bol field in pattern buffer: Match-beginning-of-line Operatornum_regs in struct re_registers: Using Registersrange argument to re_search: GNU SearchingRE_BACKSLASH_ESCAPE_IN_LIST: Syntax BitsRE_BK_PLUS_QM: Syntax BitsRE_CHAR_CLASSES: Syntax BitsRE_CONTEXT_INDEP_ANCHORS: Syntax BitsRE_CONTEXT_INDEP_ANCHORS (and ^): Match-beginning-of-line OperatorRE_CONTEXT_INDEP_OPS: Syntax BitsRE_CONTEXT_INVALID_OPS: Syntax BitsRE_DOT_NEWLINE: Syntax BitsRE_DOT_NOT_NULL: Syntax BitsRE_INTERVALS: Syntax BitsRE_LIMITED_OPS: Syntax BitsRE_NEWLINE_ALT: Syntax BitsRE_NO_BK_BRACES: Syntax BitsRE_NO_BK_PARENS: Syntax BitsRE_NO_BK_REFS: Syntax BitsRE_NO_BK_VBAR: Syntax BitsRE_NO_EMPTY_RANGES: Syntax Bitsre_nsub field, set by re_compile_pattern: GNU Regular Expression Compilingre_pattern_buffer definition: GNU Pattern Buffersre_registers: Using Registersre_syntax_options initialization: GNU Regular Expression CompilingRE_UNMATCHED_RIGHT_PAREN_ORD: Syntax BitsREG_EXTENDED: POSIX Regular Expression CompilingREG_ICASE: POSIX Regular Expression CompilingREG_NEWLINE: POSIX Regular Expression CompilingREG_NOSUB: POSIX Regular Expression Compilingregex.c: Overviewregex.h: Overviewregmatch_t: Using Byte Offsetsregs_allocated: Using RegistersREGS_FIXED: Using RegistersREGS_REALLOCATE: Using RegistersREGS_UNALLOCATED: Using Registersstart argument to re_search: GNU Searchingstart in struct re_registers: Using Registersstruct re_pattern_buffer definition: GNU Pattern Bufferssyntax field, set by re_compile_pattern: GNU Regular Expression Compilingtranslate initialization: GNU Regular Expression Compilingused field, set by re_compile_pattern: GNU Regular Expression Compiling|: Alternation Operator.)
| or \|)
[ ... ] and [^ ... ])
( ... ) or \( ... \))
\b)
\B)
\<)
\>)
\w)
\W)
[1] 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 List Operators). 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.
[2] 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.
[3] 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.
[4] Regular expressions are also referred to as “patterns,” hence the name “pattern buffer.”
[5] A table that maps all uppercase letters to the corresponding lowercase ones would work just as well for this purpose.