Regular expression matching allows you to test whether a string fits into a specific syntactic shape. You can also search a string for a substring that fits a pattern. See also the built-in function match()/rmatch().
A regular expression describes a set of strings. The simplest case is one that describes a particular string; for example, the string `foo' when regarded as a regular expression matches `foo' and nothing else. Nontrivial regular expressions use certain special constructs so that they can match more than one string. For example, the regular expression `foo%|bar' matches either the string `foo' or the string `bar'; the regular expression `c[ad]*r' matches any of the strings `cr', `car', `cdr', `caar', `cadddar' and all other such strings with any number of `a''s and `d''s.
Regular expressions have a syntax in which a few characters are special constructs and the rest are "ordinary". An ordinary character is a simple regular expression that matches that character and nothing else. The special characters are `$', `^', `.', `*', `+', `?', `[', `]' and `%'. Any other character appearing in a regular expression is ordinary, unless a `%' precedes it.
For example, `f' is not a special character, so it is ordinary, and therefore `f' is a regular expression that matches the string `f' and no other string. (It does *not*, for example, match the string `ff'.) Likewise, `o' is a regular expression that matches only `o'.
Any two regular expressions A and B can be concatenated. The result is a regular expression which matches a string if A matches some amount of the beginning of that string and B matches the rest of the string.
As a simple example, we can concatenate the regular expressions `f' and `o' to get the regular expression `fo', which matches only the string `fo'. Still trivial.
The following are the characters and character sequences that have special meaning within regular expressions. Any character not mentioned here is not special; it stands for exactly itself for the purposes of searching and matching.
`.' is a special character that matches any single character. Using
concatenation, we can make regular expressions like `a.b', which matches
any three-character string that begins with `a' and ends with `b'.
`*' is not a construct by itself; it is a suffix that means that the preceding
regular expression is to be repeated as many times as possible. In `fo*',
the `*' applies to the `o', so `fo*' matches `f' followed by any number of
`o''s.
The case of zero `o''s is allowed: `fo*' does match `f'.
`*' always applies to the *smallest* possible preceding expression. Thus,
`fo*' has a repeating `o', not a repeating `fo'.
The matcher processes a `*' construct by matching, immediately, as many
repetitions as can be found. Then it continues with the rest of the
pattern. If that fails, it backtracks, discarding some of the matches of
the `*''d construct in case that makes it possible to match the rest of
the pattern. For example, matching `c[ad]*ar' against the string
`caddaar', the `[ad]*' first matches `addaa', but this does not allow the
next `a' in the pattern to match. So the last of the matches of `[ad]' is
undone and the following `a' is tried again. Now it succeeds.
`+' is like `*' except that at least one match for the preceding pattern is
required for `+'. Thus, `c[ad]+r' does not match `cr' but does match
anything else that `c[ad]*r' would match.
`?' is like `*' except that it allows either zero or one match for the
preceding pattern. Thus, `c[ad]?r' matches `cr' or `car' or `cdr', and
nothing else.
`[ ... ]'
`[' begins a "character set", which is terminated by a `]'. In the
simplest case, the characters between the two brackets form the set.
Thus, `[ad]' matches either `a' or `d', and `[ad]*' matches any string of
`a''s and `d''s (including the empty string), from which it follows that
`c[ad]*r' matches `car', etc.
Character ranges can also be included in a character set, by writing two
characters with a `-' between them. Thus, `[a-z]' matches any lower-case
letter. Ranges may be intermixed freely with individual characters, as in
`[a-z$%.]', which matches any lower case letter or `$', `%' or period.
Note that the usual special characters are not special any more inside a
character set. A completely different set of special characters exists
inside character sets: `]', `-' and `^'.
To include a `]' in a character set, you must make it the first character.
For example, `[]a]' matches `]' or `a'. To include a `-', you must use it
in a context where it cannot possibly indicate a range: that is, as the
first character, or immediately after a range.
`[^ ... ]'
`[^' begins a "complement character set", which matches any character
except the ones specified. Thus, `[^a-z0-9A-Z]' matches all characters
*except* letters and digits.
`^' is not special in a character set unless it is the first character.
The character following the `^' is treated as if it were first (it may be
a `-' or a `]').
`^' is a special character that matches the empty string -- but only if at the
beginning of the string being matched. Otherwise it fails to match
anything. Thus, `^foo' matches a `foo' which occurs at the beginning of
the string.
`$' is similar to `^' but matches only at the *end* of the string. Thus,
`xx*$' matches a string of one or more `x''s at the end of the string.
`%' has two functions: it quotes the above special characters (including `%'),
and it introduces additional special constructs.
Because `%' quotes special characters, `%$' is a regular expression that
matches only `$', and `%[' is a regular expression that matches only `[',
and so on.
For the most part, `%' followed by any character matches only that
character. However, there are several exceptions: characters that, when
preceded by `%', are special constructs. Such characters are always
ordinary when encountered on their own.
No new special characters will ever be defined. All extensions to the
regular expression syntax are made by defining new two-character
constructs that begin with `%'.
`%|' specifies an alternative. Two regular expressions A and B with `%|' in
between form an expression that matches anything that either A or B will
match.
Thus, `foo%|bar' matches either `foo' or `bar' but no other string.
`%|' applies to the largest possible surrounding expressions. Only a
surrounding `%( ... %)' grouping can limit the grouping power of `%|'.
Full backtracking capability exists for when multiple `%|''s are used.
`%( ... %)'
is a grouping construct that serves three purposes:
To enclose a set of `%|' alternatives for other operations. Thus,
`%(foo%|bar%)x' matches either `foox' or `barx'.
To enclose a complicated expression for a following `*', `+', or `?'
to operate on. Thus, `ba%(na%)*' matches `bananana', etc., with any
number of `na''s, including none.
To mark a matched substring for future reference.
This last application is not a consequence of the idea of a parenthetical
grouping; it is a separate feature that happens to be assigned as a second
meaning to the same `%( ... %)' construct because there is no conflict in
practice between the two meanings. Here is an explanation of this
feature:
`%DIGIT'
After the end of a `%( ... %)' construct, the matcher remembers the
beginning and end of the text matched by that construct. Then, later on
in the regular expression, you can use `%' followed by DIGIT to mean
"match the same text matched by the DIGIT'th `%( ... %)' construct in the
pattern." The `%( ... %)' constructs are numbered in the order that their
`%(''s appear in the pattern.
The strings matching the first nine `%( ... %)' constructs appearing in a
regular expression are assigned numbers 1 through 9 in order of their
beginnings. `%1' through `%9' may be used to refer to the text matched by
the corresponding `%( ... %)' construct.
For example, `%(.*%)%1' matches any string that is composed of two
identical halves. The `%(.*%)' matches the first half, which may be
anything, but the `%1' that follows must match the same exact text.
`%b' matches the empty string, but only if it is at the beginning or end of a
word. Thus, `%bfoo%b' matches any occurrence of `foo' as a separate word.
`%bball%(s%|%)%b' matches `ball' or `balls' as a separate word.
For the purposes of this construct and the five that follow, a word is
defined to be a sequence of letters and/or digits.
`%B' matches the empty string, provided it is *not* at the beginning or end of
a word.
`%<' matches the empty string, but only if it is at the beginning of a word.
`%>' matches the empty string, but only if it is at the end of a word.
`%w' matches any word-constituent character (i.e., any letter or digit).
`%W' matches any character that is not a word constituent.