| REGEX(3) | Library Functions Manual | REGEX(3) | 
regcomp, regexec,
  regerror, regfree,
  regasub, regnsub —
#include <regex.h>
int
  
  regcomp(regex_t * restrict preg,
    const char * restrict pattern, 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);
ssize_t
  
  regnsub(char
    *buf, size_t
    bufsiz, const char
    *sub, const regmatch_t
    *rm, const char
    *str);
ssize_t
  
  regasub(char
    **buf, const char
    *sub, const regmatch_t
    *rm, const char
    *sstr);
regcomp() function 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-allocated 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 functions, a type
    regoff_t, and a number of constants with names
    starting with “REG_”.
The regcomp() function 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. The cflags argument is the
    bitwise OR of zero or more of the following flags:
REG_EXTENDEDREG_BASICREG_EXTENDED to improve readability.REG_NOSPECREG_EXTENDED and
      REG_NOSPEC may not be used in the same call to
      regcomp().REG_ICASEREG_NOSUBREG_NEWLINE[^’
      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 ‘$’ anchor matches
      the null string before any newline in the string in addition to its normal
      function.REG_PENDREG_GNUN where
          N is a single digit number between
          1 and 9.This is an extension, compatible with but not specified by IEEE Std 1003.2 (“POSIX.2”), and should be used with caution in software intended to be portable to other systems.
When successful, regcomp() returns 0 and
    fills in the structure pointed to by preg. One member
    of that structure (other than re_endp) is publicized:
    re_nsub, of type size_t,
    contains the number of parenthesized subexpressions within the RE (except
    that the value of this member is undefined if the
    REG_NOSUB flag was used). If
    regcomp() fails, it returns a non-zero error code;
    see DIAGNOSTICS.
The regexec() function matches the
    compiled RE pointed to by preg against the
    string, subject to the flags in
    eflags, and reports results using
    nmatch, pmatch, and the returned
    value. The RE must have been compiled by a previous invocation of
    regcomp(). The compiled form is not altered during
    execution of regexec(), so a single compiled RE can
    be used simultaneously by multiple threads.
By default, the NUL-terminated string pointed to by string is considered to be the text of an entire line, minus any terminating newline. The eflags argument is the bitwise OR of zero or more of the following flags:
REG_NOTBOL^’,
      ‘[[:<:]]’, and
      ‘\<’ do not match before it; but
      see REG_STARTEND below. This does not affect the
      behavior of newlines under REG_NEWLINE.REG_NOTEOL$’ anchor does not match before it.
      This does not affect the behavior of newlines under
      REG_NEWLINE.REG_STARTENDWithout REG_NOTBOL, the position
        rm_so is considered the beginning of a line, such
        that ‘^’ matches before it, and
        the beginning of a word if there is a word character at this position,
        such that ‘[[:<:]]’ and
        ‘\<’ match before it.
With REG_NOTBOL, the character at
        position rm_so is treated as the continuation of a
        line, and if rm_so is greater than 0, the
        preceding character is taken into consideration. If the preceding
        character is a newline and the regular expression was compiled with
        REG_NEWLINE,
        ‘^’ matches before the string; if
        the preceding character is not a word character but the string starts
        with a word character,
        ‘[[:<:]]’ and
        ‘\<’ match before the
      string.
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() ignores 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 structure 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
    parenthesized 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 example 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 output 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().
The regerror() function 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. The (regerror() may
    be able to supply a more detailed message using information from the
    regex_t.) The regerror()
    function 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 will not
    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.
The regfree() function 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.
The regnsub() and
    regasub() functions perform substitutions using
    sed(1) like syntax. They return
    the length of the string that would have been created if there was enough
    space or -1 on error, setting
    errno. The result is being placed in
    buf which is user-supplied in
    regnsub() and dynamically allocated in
    regasub(). The sub argument
    contains a substitution string which might refer to the first 9 regular
    expression strings using “\<n>” to refer to the nth
    matched item, or “&” (which is equivalent to
    “\0”) to refer to the full match. The rm
    array must be at least 10 elements long, and should contain the result of
    the matches from a previous regexec() call. Only 10
    elements of the rm array can be used. The
    str argument contains the source string to apply the
    transformation to.
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 ordinary 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 legal 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.
regcomp() and
  regexec() include the following:
REG_NOMATCHregexec() function failed to matchREG_BADPATREG_ECOLLATEREG_ECTYPEREG_EESCAPE\’ applied to unescapable
    characterREG_ESUBREGREG_EBRACK[ ]’ not balancedREG_EPAREN( )’ not balancedREG_EBRACE{ }’ not balancedREG_BADBR{
    }’REG_ERANGE[
    ]’REG_ESPACEREG_BADRPT?’,
      ‘*’, or
      ‘+’ operand invalidREG_EMPTYREG_ASSERTREG_INVARGREG_ILLSEQIEEE Std 1003.2 (“POSIX.2”), sections 2.8 (Regular Expression Notation) and B.5 (C Binding for Regular Expression Matching).
The regnsub() and
    regasub() functions appeared in
    NetBSD 8.
The back-reference code is subtle and doubts linger about its correctness in complex cases.
The regexec() function performance is
    poor. This will improve with later releases. The
    nmatch argument exceeding 0 is expensive;
    nmatch exceeding 1 is worse. The
    regexec() function is largely insensitive to RE
    complexity except that back references are massively
    expensive. RE length does matter; in particular, there is a strong speed
    bonus for keeping RE length under about 30 characters, with most special
    characters counting roughly double.
The regcomp() function 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 machine 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 cannot 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 combinations of word-boundary matching and anchoring.
Word-boundary matching does not work properly in multibyte locales.
| March 11, 2021 | NetBSD 10.0 |