A regular expression is a group of symbols which describe a unique string of characters. An example of a simple regular expression is the word "donkey". In a group of words, the regular expression "donkey" matches only other instances of the word "donkey" and nothing else. So if we had a text file and did a search for "donkey" every time that word appeared in the text it would show up in our search. This definition can be expanded if we define the period "." as a wild card replacement for a single character. Then the regular expression ".onkey" would include "honkey" or "tonkey" or "bonkey" as well as "donkey". We can continue to expand on the definition by adding the asterisk to the period " .* " as a substitute for any number of characters. Then the regular expression "don.* " will not only include "donkey" but will include all strings of characters beginning with "don" including "don" itself. A search of a text file for "don.* " will turn up dongle, donkey, donner, dondoodit, dondiddle, etc. Regular expression matching, therefore, puts together a whole set of rules that allow 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, and just as importantly, you can replace one string with another.
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 which 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 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 which 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 anything except a newline. Using concatenation, we can make regular expressions like `a.b' which matches any three-character string which begins with `a' and ends with `b'.
is not a construct by itself; it is a suffix, which means 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, backtracking occurs, 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 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 `$', `%' or period.
Note that the usual special characters are not special any more inside a character set. A completely different set of special characters exists inside character sets: `]', `-' and `^'. To include a `]' in a character set, you must make it the first character. For example, `[]a]' matches `]' or `a'. To include a `-', you must use it in a context where it cannot possibly indicate a range: that is, as the first character, or immediately after a range.
`[^' begins a "complement character set", which matches any character except the ones specified. Thus, `[^a-z0-9A-Z]' matches all characters except letters and digits. Note that the ^ has to be within brackets. Outside of brackets it has a different meaning as mentioned below. `^' is not special in a character set unless it is the first character. The character following the `^' is treated as if it were first (it may be a `-' or a `]').
is a special character that matches the empty string -- but only if at the beginning of a line in the text being matched. Otherwise it fails to match anything. Thus,`^foo' matches a `foo' which occurs at the beginning of a line.
is similar to `^' but matches only at the end of a line. Thus, `xx*$' matches a string of one or more `x''s at the end of a line.
has two functions: it quotes the above special characters (including `\'), and it introduces additional special constructs. If you wanted to search for the dollar sign within a text you would have to use \$ in place of just $ since the dollar sign has a special meaning. Because `\' quotes special characters, `\$' is a regular expression which matches only `$', and `\[' is a regular expression which matches only `[', and so on.
For the most part, `\' followed by any character matches only that character.However, there are several exceptions: characters which, 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 when multiple `\|''s are used.
is a grouping construct that serves three purposes:
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 the DIGIT'th time by the `\( ... \)' construct." The `\( ... \)' constructs are numbered in order of commencement in the regexp.
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.
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.
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.
matches any word-constituent character.
matches any character that is not a word-constituent. What the `\( ... \)' groupings matched.