The Fn 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.
The Fn regexec function matches a NUL -terminated Fa string against the compiled regular expression in Fa prog . It returns 1 for success and 0 for failure, and adjusts the contents of Fa prog Ns '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 Fn regexec has been done using the Fn regexp , each startp - endp pair describes one substring within the Fa 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 Fa 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 Fn regsub function copies Fa source to Fa dest , making substitutions according to the most recent Fn regexec performed using Fa prog . Each instance of `&' in Fa source is replaced by the substring indicated by startp Bq and endp Bq . Each instance of `\ n , ' where n is a digit, is replaced by the substring indicated by startp Bq n and endp Bq n . To get a literal `&' or `\ n ' into Fa dest , prefix it with `\'; to get a literal `\' preceding `&' or `\ n , ' prefix it with another `\'.
The Fn regerror function is called whenever an error is detected in Fn regcomp , Fn regexec , or Fn regsub . The default Fn regerror writes the string Fa msg , with a suitable indicator of origin, on the standard error output and invokes exit(2). The Fn 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.)
The restriction against applying `*' or `+' to a possibly-null operand is an artifact of the simplistic implementation.
Does not support egrepNs'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.