| REGEXP(3) | Library Functions Manual | REGEXP(3) | 
regcomp, regexec,
  regsub, regerror —
#include <regexp.h>
regexp *
  
  regcomp(const
    char *exp);
int
  
  regexec(const
    regexp *prog, const char
    *string);
void
  
  regsub(const
    regexp *prog, const char
    *source, char
    *dest);
void
  
  regerror(const
    char *msg);
The regcomp(),
    regexec(), regsub(), and
    regerror() functions implement
    egrep(1)-style regular
    expressions and supporting facilities.
The regcomp() function compiles a regular
    expression into a structure of type regexp, and returns a
    pointer to it. The space has been allocated using
    malloc(3) and may be released
    by free(3).
The regexec() function matches a
    NUL-terminated string against
    the compiled regular expression in prog. It returns 1
    for success and 0 for failure, and adjusts the contents of
    prog's startp and
    endp (see below) accordingly.
The members of a regexp structure include at least the following (not necessarily in order):
char *startp[NSUBEXP]; char *endp[NSUBEXP];
where NSUBEXP is defined (as 10) in the
    header file. Once a successful regexec() has been
    done using the regexp(), each
    startp- endp pair describes one
    substring within the string, with the
    startp pointing to the first character of the substring
    and the endp pointing to the first character following the
    substring. The 0th substring is the substring of
    string that matched the whole regular expression. The
    others are those substrings that matched parenthesized expressions within
    the regular expression, with parenthesized expressions numbered in
    left-to-right order of their opening parentheses.
The regsub() function copies
    source to dest, making
    substitutions according to the most recent regexec()
    performed using prog. Each instance of `&' in
    source is replaced by the substring indicated by
    startp[] and endp[]. Each instance of
    ‘\n’, where n is a digit,
    is replaced by the substring indicated by
    startp[n] and
    endp[n]. To get a literal `&' or
    ‘\n’ into dest, prefix
    it with `\'; to get a literal `\' preceding `&' or
    ‘\n’, prefix it with another `\'.
The regerror() function is called whenever
    an error is detected in regcomp(),
    regexec(), or regsub(). The
    default regerror() writes the string
    msg, with a suitable indicator of origin, on the
    standard error output and invokes
    exit(3). The
    regerror() function can be replaced by the user if
    other actions are desirable.
A branch is zero or more pieces, concatenated. It matches a match for the first, followed by a match for the second, etc.
A piece is an atom possibly followed by `*', `+', or `?'. An atom followed by `*' matches a sequence of 0 or more matches of the atom. An atom followed by `+' matches a sequence of 1 or more matches of the atom. An atom followed by `?' matches a match of the atom, or the null string.
An atom is a regular expression in parentheses (matching a match for the regular expression), a range (see below), `.' (matching any single character), `^' (matching the null string at the beginning of the input string), `$' (matching the null string at the end of the input string), a `\' followed by a single character (matching that character), or a single character with no other significance (matching that character).
A range is a sequence of characters enclosed in `[]'. It normally matches any single character from the sequence. If the sequence begins with `^', it matches any single character not from the rest of the sequence. If two characters in the sequence are separated by `-', this is shorthand for the full list of ASCII characters between them (e.g. `[0-9]' matches any decimal digit). To include a literal `]' in the sequence, make it the first character (following a possible `^'). To include a literal `-', make it the first or last character.
In general, the possibilities in a list of branches are considered in left-to-right order, the possibilities for `*', `+', and `?' are considered longest-first, nested constructs are considered from the outermost in, and concatenated constructs are considered leftmost-first. The match that will be chosen is the one that uses the earliest possibility in the first choice that has to be made. If there is more than one choice, the next will be made in the same manner (earliest possibility) subject to the decision on the first choice. And so forth.
For example, ‘(ab|a)b*c’
    could match `abc' in one of two ways. The first choice is between `ab' and
    `a'; since `ab' is earlier, and does lead to a successful overall match, it
    is chosen. Since the `b' is already spoken for, the `b*' must match its last
    possibility—the empty string—since it must respect the earlier
    choice.
In the particular case where no `|'s are present and there is only
    one `*', `+', or `?', the net effect is that the longest possible match will
    be chosen. So ‘ab*’, presented with
    `xabbbby', will match `abbbb'. Note that if
    ‘ab*’, is tried against `xabyabbbz',
    it will match `ab' just after `x', due to the begins-earliest rule. (In
    effect, the decision on where to start the match is the first choice to be
    made, hence subsequent choices must respect it even if this leads them to
    less-preferred alternatives.)
regcomp() function returns
  NULL for a failure (regerror()
  permitting), where failures are syntax errors, exceeding implementation
  limits, or applying `+' or `*' to a possibly-null operand.
regcomp(),
  regexec(), regsub(), and
  regerror() were written at the University of Toronto
  and appeared in 4.3BSD-Tahoe. They are intended to be
  compatible with the Bell V8
  regexp(3), but are not derived
  from Bell code.
The restriction against applying `*' or `+' to a possibly-null operand is an artifact of the simplistic implementation.
Does not support egrep(1)'s newline-separated branches; neither does the V8 regexp(3), though.
Due to emphasis on compactness and simplicity, it's not strikingly fast. It does give special attention to handling simple cases quickly.
| June 4, 1993 | NetBSD 10.0 |