     REGEX(3)                                                           REGEX(3)
                                   March 20, 1994

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

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

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

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

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

          void regfree(regex_t *preg);

     DESCRIPTION
          These routines implement POSIX 1003.2 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-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_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 POSIX
                        1003.2, 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.

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     REGEX(3)                                                           REGEX(3)
                                   March 20, 1994

          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 POSIX 1003.2, 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.

          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_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 newlines under
                        REG_NEWLINE.

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     REGEX(3)                                                           REGEX(3)
                                   March 20, 1994

          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 POSIX 1003.2, and should be used with
                        caution 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 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.)

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     REGEX(3)                                                           REGEX(3)
                                   March 20, 1994

          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 POSIX
          1003.2, 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 1003.2 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 matching.

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     REGEX(3)                                                           REGEX(3)
                                   March 20, 1994

          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 1003.2 (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.

     SEE ALSO
          grep(1), re_format(7)

          POSIX 1003.2, 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

                                        - 5 -          Formatted:  July 13, 2000

     REGEX(3)                                                           REGEX(3)
                                   March 20, 1994

          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.

          There is one known functionality bug.  The implementation of
          internationalization is incomplete: the locale is always assumed to be
          the default one of 1003.2, and only the collating elements etc. of
          that locale are available.

          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 1003.2, 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.

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     REGEX(3)                                                           REGEX(3)
                                   March 20, 1994

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

                                        - 7 -          Formatted:  July 13, 2000