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REGEX(3)                   Library Functions Manual                   REGEX(3)

     regcomp, regexec, regerror, regfree, regasub, regnsub - regular-
     expression library

     Standard C Library (libc, -lc)

     #include <regex.h>

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

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

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

     regfree(regex_t *preg);

     regnsub(char *buf, size_t bufsiz, const char *sub, const regmatch_t *rm,
         const char *str);

     regasub(char **buf, const char *sub, const regmatch_t *rm,
         const char *sstr);

     These routines implement IEEE Std 1003.2 ("POSIX.2") regular expressions
     ("RE"s); see re_format(7).  The 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_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 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.  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 character 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.

     REG_GNU       Include GNU-inspired extensions:

                         \N  Use backreference N where N is a single digit
                             number between 1 and 9.
                         \a  Visual Bell
                         \b  Match a position that is a word boundary.
                         \B  Match a position that is a not word boundary.
                         \f  Form Feed
                         \n  Line Feed
                         \r  Carriage return
                         \s  Alias for [[:space:]]
                         \S  Alias for [^[:space:]]
                         \t  Horizontal Tab
                         \v  Vertical Tab
                         \w  Alias for [[:alnum:]_]
                         \W  Alias for [^[:alnum:]_]
                         \'  Matches the end of the subject string (the string
                             to be matched).
                         \`  Matches the beginning of the subject string.

                   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

     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    The first character of the string is treated as the
                   continuation of a line.  This means that the anchors `^',
                   `[[:<:]]', and `\<' do not match before it; but see
                   REG_STARTEND below.  This does not affect the behavior of
                   newlines under REG_NEWLINE.

     REG_NOTEOL    The NUL terminating the string does not end a line, so the
                   `$' anchor does 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 end before the byte located at
                   string + pmatch[0].rm_eo, 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
                   caution in software intended to be portable to other

                   Without 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

                   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

     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

     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

     There are a number of decisions that IEEE Std 1003.2 ("POSIX.2") leaves
     up to the implementor, 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

     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

     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.

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

     REG_NOMATCH   The regexec() function 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    cannot happen - you found a bug
     REG_INVARG    invalid argument, e.g. negative-length string
     REG_ILLSEQ    illegal byte sequence (bad multibyte character)

     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).

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

     The regnsub() and regasub() functions appeared in NetBSD 8.

     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.

     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.

NetBSD 10.99                    March 11, 2021                    NetBSD 10.99