| REGEX(3) | Library Functions Manual | REGEX(3) | 
regex, 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[], 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() 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_''.
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_EXTENDEDREG_BASICREG_NOSPECREG_EXTENDED and
      REG_NOSPEC may not be used in the same call to
      regcomp().REG_ICASEREG_NOSUBREG_NEWLINEREG_PENDWhen 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.
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 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_NOTBOLREG_NEWLINE.REG_NOTEOLREG_NEWLINE.REG_STARTENDREG_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() 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().
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 1003.2-1992
    (“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.
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 1003.2-1992 (“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() failed to matchREG_BADPATREG_ECOLLATEREG_ECTYPEREG_EESCAPEREG_ESUBREGREG_EBRACKREG_EPARENREG_EBRACEREG_BADBRREG_ERANGEREG_ESPACEREG_BADRPTREG_EMPTYREG_ASSERTREG_INVARGIEEE Std 1003.2-1992 (“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.
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 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.
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 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 1003.2-1992 (“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 combinations of word-boundary matching and anchoring.
| February 26, 2018 | NetBSD 9.4 |