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Mac Magazin/MacEasy 34
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Mac Magazin and MacEasy Magazine CD - Issue 34.iso
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Alpha ƒ
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Regular Expressions
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1996-03-02
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NOTE: The following is the manual page supplied with the regular expression
routines used both in Alpha and Tcl. - pete.
NAME
regcomp, regexec, regsub, regerror - regular expression
handler
SYNOPSIS
#include <regexp.h>
regexp *regcomp(exp)
char *exp;
int regexec(prog, string)
regexp *prog;
char *string;
regsub(prog, source, dest)
regexp *prog;
char *source;
char *dest;
regerror(msg)
char *msg;
DESCRIPTION
These functions implement egrep(1)-style regular expressions
and supporting facilities.
Regcomp 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.
Regexec 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.
Regsub 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[0] and endp[0]. 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 `\'.
Regerror 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(2). Regerror can be replaced
by the user if other actions are desirable.
REGULAR EXPRESSION SYNTAX
A regular expression is zero or more branches, separated by
`|'. It matches anything that matches one of the branches.
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.
AMBIGUITY
If a regular expression could match two different parts of
the input string, it will match the one which begins
earliest. If both begin in the same place but match
different lengths, or match the same length in different
ways, life gets messier, as follows.
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.)
In addition, \w matches an alphanumeric character (including "_")
and \W a nonalphanumeric. Word boundaries may be matched by \b,
and non-boundaries by \B. A whi- tespace character is matched by
\s, non-whitespace by \S. A numeric character is matched by \d,
non-numeric by \D. You may use \w, \s and \d within character
classes.
The class of character recognized by \w (and hence not recognized
by \W), can be augmented via the 'addAlphaChars' command.
DIAGNOSTICS
Regcomp returns NULL for a failure (regerror permitting),
where failures are syntax errors, exceeding implementation
limits, or applying `+' or `*' to a possibly-null operand.
HISTORY
Both code and manual page were written at U of T. They are
intended to be compatible with the Bell V8 regexp(3), but
are not derived from Bell code.
BUGS
Empty branches and empty regular expressions are not
portable to V8.
The restriction against applying `*' or `+' to a possibly-
null operand is an artifact of the simplistic
implementation.
Does not support egrep'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.