path: root/newlib/libc/posix/regex.3

REGEX(3)                           BSD Library Functions Manual                          REGEX(3)

NAME
     regcomp, regexec, regerror, regfree -- regular-expression library

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <regex.h>

     int
     regcomp(regex_t *restrict preg, const char *_restrictpattern, int cflags);

     int
     regexec(const regex_t *_restrict preg, const char *_restrict string, size_t nmatch,
         regmatch_t pmatch[_restrict], int eflags);

     size_t
     regerror(int errcode, const regex_t *_restrict preg, char *_restrict errbuf,
         size_t errbuf_size);

     void
     regfree(regex_t *preg);

DESCRIPTION
     These routines implement IEEE Std 1003.2 ("POSIX.2") regular expressions ("RE"s); see
     re_format(7).  Regcomp() compiles an RE written as a string into an internal form, regexec()
     matches that internal form against a string and reports results, regerror() transforms error
     codes from either into human-readable messages, and regfree() frees any dynamically-allo-
     cated storage used by the internal form of an RE.

     The header <regex.h> declares two structure types, regex_t and regmatch_t, the former for
     compiled internal forms and the latter for match reporting.  It also declares the four func-
     tions, a type regoff_t, and a number of constants with names starting with "REG_".

     Regcomp() compiles the regular expression contained in the pattern string, subject to the
     flags in cflags, and places the results in the regex_t structure pointed to by preg.  Cflags
     is the bitwise OR of zero or more of the following flags:

     REG_EXTENDED  Compile modern ("extended") REs, rather than the obsolete ("basic") REs that
                   are the default.

     REG_BASIC     This is a synonym for 0, provided as a counterpart to REG_EXTENDED to improve
                   readability.

     REG_NOSPEC    Compile with recognition of all special characters turned off.  All characters
                   are thus considered ordinary, so the "RE" is a literal string.  This is an ex-
                   tension, compatible with but not specified by IEEE Std 1003.2 ("POSIX.2"), and
                   should be used with caution in software intended to be portable to other sys-
                   tems.  REG_EXTENDED and REG_NOSPEC may not be used in the same call to
                   regcomp().

     REG_ICASE     Compile for matching that ignores upper/lower case distinctions.  See
                   re_format(7).

     REG_NOSUB     Compile for matching that need only report success or failure, not what was
                   matched.

     REG_NEWLINE   Compile for newline-sensitive matching.  By default, newline is a completely
                   ordinary character with no special meaning in either REs or strings.  With
                   this flag, '[^' bracket expressions and '.' never match newline, a '^' anchor
                   matches the null string after any newline in the string in addition to its
                   normal function, and the '$' anchor matches the null string before any newline
                   in the string in addition to its normal function.

     REG_PEND      The regular expression ends, not at the first NUL, but just before the charac-
                   ter pointed to by the re_endp member of the structure pointed to by preg.  The
                   re_endp member is of type const char *.  This flag permits inclusion of NULs
                   in the RE; they are considered ordinary characters.  This is an extension,
                   compatible with but not specified by IEEE Std 1003.2 ("POSIX.2"), and should
                   be used with caution in software intended to be portable to other systems.

     When successful, regcomp() returns 0 and fills in the structure pointed to by preg.  One
     member of that structure (other than re_endp) is publicized: re_nsub, of type size_t, con-
     tains the number of parenthesized subexpressions within the RE (except that the value of
     this member is undefined if the REG_NOSUB flag was used).  If regcomp() fails, it returns a
     non-zero error code; see DIAGNOSTICS.

     Regexec() matches the compiled RE pointed to by preg against the string, subject to the
     flags in eflags, and reports results using nmatch, pmatch, and the returned value.  The RE
     must have been compiled by a previous invocation of regcomp().  The compiled form is not al-
     tered during execution of regexec(), so a single compiled RE can be used simultaneously by
     multiple threads.

     By default, the NUL-terminated string pointed to by string is considered to be the text of
     an entire line, minus any terminating newline.  The eflags argument is the bitwise OR of
     zero or more of the following flags:

     REG_NOTBOL    The first character of the string is not the beginning of a line, so the '^'
                   anchor should not match before it.  This does not affect the behavior of new-
                   lines under REG_NEWLINE.

     REG_NOTEOL    The NUL terminating the string does not end a line, so the '$' anchor should
                   not match before it.  This does not affect the behavior of newlines under
                   REG_NEWLINE.

     REG_STARTEND  The string is considered to start at string + pmatch[0].rm_so and to have a
                   terminating NUL located at string + pmatch[0].rm_eo (there need not actually
                   be a NUL at that location), regardless of the value of nmatch.  See below for
                   the definition of pmatch and nmatch.  This is an extension, compatible with
                   but not specified by IEEE Std 1003.2 ("POSIX.2"), and should be used with cau-
                   tion in software intended to be portable to other systems.  Note that a non-
                   zero rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
                   of the string, not how it is matched.

     See re_format(7) for a discussion of what is matched in situations where an RE or a portion
     thereof could match any of several substrings of string.

     Normally, regexec() returns 0 for success and the non-zero code REG_NOMATCH for failure.
     Other non-zero error codes may be returned in exceptional situations; see DIAGNOSTICS.

     If REG_NOSUB was specified in the compilation of the RE, or if nmatch is 0, regexec() ig-
     nores the pmatch argument (but see below for the case where REG_STARTEND is specified).
     Otherwise, pmatch points to an array of nmatch structures of type regmatch_t.  Such a struc-
     ture has at least the members rm_so and rm_eo, both of type regoff_t (a signed arithmetic
     type at least as large as an off_t and a ssize_t), containing respectively the offset of the
     first character of a substring and the offset of the first character after the end of the
     substring.  Offsets are measured from the beginning of the string argument given to
     regexec().  An empty substring is denoted by equal offsets, both indicating the character
     following the empty substring.

     The 0th member of the pmatch array is filled in to indicate what substring of string was
     matched by the entire RE.  Remaining members report what substring was matched by parenthe-
     sized subexpressions within the RE; member i reports subexpression i, with subexpressions
     counted (starting at 1) by the order of their opening parentheses in the RE, left to right.
     Unused entries in the array (corresponding either to subexpressions that did not participate
     in the match at all, or to subexpressions that do not exist in the RE (that is, i >
     preg->re_nsub)) have both rm_so and rm_eo set to -1.  If a subexpression participated in the
     match several times, the reported substring is the last one it matched.  (Note, as an exam-
     ple in particular, that when the RE '(b*)+' matches 'bbb', the parenthesized subexpression
     matches each of the three 'b's and then an infinite number of empty strings following the
     last 'b', so the reported substring is one of the empties.)

     If REG_STARTEND is specified, pmatch must point to at least one regmatch_t (even if nmatch
     is 0 or REG_NOSUB was specified), to hold the input offsets for REG_STARTEND.  Use for out-
     put is still entirely controlled by nmatch; if nmatch is 0 or REG_NOSUB was specified, the
     value of pmatch[0] will not be changed by a successful regexec().

     Regerror() maps a non-zero errcode from either regcomp() or regexec() to a human-readable,
     printable message.  If preg is non-NULL, the error code should have arisen from use of the
     regex_t pointed to by preg, and if the error code came from regcomp(), it should have been
     the result from the most recent regcomp() using that regex_t.  (Regerror() may be able to
     supply a more detailed message using information from the regex_t.)  Regerror() places the
     NUL-terminated message into the buffer pointed to by errbuf, limiting the length (including
     the NUL) to at most errbuf_size bytes.  If the whole message won't fit, as much of it as
     will fit before the terminating NUL is supplied.  In any case, the returned value is the
     size of buffer needed to hold the whole message (including terminating NUL).  If errbuf_size
     is 0, errbuf is ignored but the return value is still correct.

     If the errcode given to regerror() is first ORed with REG_ITOA, the "message" that results
     is the printable name of the error code, e.g. "REG_NOMATCH", rather than an explanation
     thereof.  If errcode is REG_ATOI, then preg shall be non-NULL and the re_endp member of the
     structure it points to must point to the printable name of an error code; in this case, the
     result in errbuf is the decimal digits of the numeric value of the error code (0 if the name
     is not recognized).  REG_ITOA and REG_ATOI are intended primarily as debugging facilities;
     they are extensions, compatible with but not specified by IEEE Std 1003.2 ("POSIX.2"), and
     should be used with caution in software intended to be portable to other systems.  Be warned
     also that they are considered experimental and changes are possible.

     Regfree() frees any dynamically-allocated storage associated with the compiled RE pointed to
     by preg.  The remaining regex_t is no longer a valid compiled RE and the effect of supplying
     it to regexec() or regerror() is undefined.

     None of these functions references global variables except for tables of constants; all are
     safe for use from multiple threads if the arguments are safe.

IMPLEMENTATION CHOICES
     There are a number of decisions that IEEE Std 1003.2 ("POSIX.2") leaves up to the implemen-
     tor, either by explicitly saying "undefined" or by virtue of them being forbidden by the RE
     grammar.  This implementation treats them as follows.

     See re_format(7) for a discussion of the definition of case-independent matching.

     There is no particular limit on the length of REs, except insofar as memory is limited.
     Memory usage is approximately linear in RE size, and largely insensitive to RE complexity,
     except for bounded repetitions.  See BUGS for one short RE using them that will run almost
     any system out of memory.

     A backslashed character other than one specifically given a magic meaning by IEEE Std 1003.2
     ("POSIX.2") (such magic meanings occur only in obsolete ["basic"] REs) is taken as an ordi-
     nary character.

     Any unmatched '[' is a REG_EBRACK error.

     Equivalence classes cannot begin or end bracket-expression ranges.  The endpoint of one
     range cannot begin another.

     RE_DUP_MAX, the limit on repetition counts in bounded repetitions, is 255.

     A repetition operator ('?', '*', '+', or bounds) cannot follow another repetition operator.
     A repetition operator cannot begin an expression or subexpression or follow '^' or '|'.

     '|' cannot appear first or last in a (sub)expression or after another '|', i.e. an operand
     of '|' cannot be an empty subexpression.  An empty parenthesized subexpression, '()', is le-
     gal and matches an empty (sub)string.  An empty string is not a legal RE.

     A '{' followed by a digit is considered the beginning of bounds for a bounded repetition,
     which must then follow the syntax for bounds.  A '{' not followed by a digit is considered
     an ordinary character.

     '^' and '$' beginning and ending subexpressions in obsolete ("basic") REs are anchors, not
     ordinary characters.

SEE ALSO
     grep(1), re_format(7)

     IEEE Std 1003.2 ("POSIX.2"), sections 2.8 (Regular Expression Notation) and B.5 (C Binding
     for Regular Expression Matching).

DIAGNOSTICS
     Non-zero error codes from regcomp() and regexec() include the following:

     REG_NOMATCH   regexec() failed to match
     REG_BADPAT    invalid regular expression
     REG_ECOLLATE  invalid collating element
     REG_ECTYPE    invalid character class
     REG_EESCAPE   '\' applied to unescapable character
     REG_ESUBREG   invalid backreference number
     REG_EBRACK    brackets '[ ]' not balanced
     REG_EPAREN    parentheses '( )' not balanced
     REG_EBRACE    braces '{ }' not balanced
     REG_BADBR     invalid repetition count(s) in '{ }'
     REG_ERANGE    invalid character range in '[ ]'
     REG_ESPACE    ran out of memory
     REG_BADRPT    '?', '*', or '+' operand invalid
     REG_EMPTY     empty (sub)expression
     REG_ASSERT    can't happen - you found a bug
     REG_INVARG    invalid argument, e.g. negative-length string

HISTORY
     Originally written by Henry Spencer.  Altered for inclusion in the 4.4BSD distribution.

BUGS
     This is an alpha release with known defects.  Please report problems.

     The back-reference code is subtle and doubts linger about its correctness in complex cases.

     Regexec() performance is poor.  This will improve with later releases.  Nmatch exceeding 0
     is expensive; nmatch exceeding 1 is worse.  Regexec() is largely insensitive to RE complex-
     ity except that back references are massively expensive.  RE length does matter; in particu-
     lar, there is a strong speed bonus for keeping RE length under about 30 characters, with
     most special characters counting roughly double.

     Regcomp() implements bounded repetitions by macro expansion, which is costly in time and
     space if counts are large or bounded repetitions are nested.  An RE like, say,
     '((((a{1,100}){1,100}){1,100}){1,100}){1,100}' will (eventually) run almost any existing ma-
     chine out of swap space.

     There are suspected problems with response to obscure error conditions.  Notably, certain
     kinds of internal overflow, produced only by truly enormous REs or by multiply nested
     bounded repetitions, are probably not handled well.

     Due to a mistake in IEEE Std 1003.2 ("POSIX.2"), things like 'a)b' are legal REs because ')'
     is a special character only in the presence of a previous unmatched '('.  This can't be
     fixed until the spec is fixed.

     The standard's definition of back references is vague.  For example, does 'a\(\(b\)*\2\)*d'
     match 'abbbd'?  Until the standard is clarified, behavior in such cases should not be relied
     on.

     The implementation of word-boundary matching is a bit of a kludge, and bugs may lurk in com-
     binations of word-boundary matching and anchoring.

BSD                                       March 20, 1994                                      BSD