28.7 Regular expression syntax

Regular expressions can be used with functions such as find-regexp-in-string, regexp-find-symbols, count-regexp-occurrences and editor:regular-expression-search and also in the LispWorks IDE.

A regular expression (regexp) allows the specification of the search string to include wild characters, repeated characters, ranges of characters, and alternatives. Strings which follow a specific pattern can be located, which makes regular expression searches very powerful.

The regular expression syntax used is similar to that of GNU Emacs. Most characters match themselves, but a regular expression can contain the following special characters to produce the search pattern:

`.`	Matches any single character except a newline. For example, `c.r` matches any three character string starting with `c` and ending with `r`.
`*`	Matches the previous regexp any number of times (including zero times). For example, `car` matches strings beginning with `c` and ending with `r`, with any number of `a`s in-between. An empty regexp followed by `` matches an empty part of the input. By extension, `^*` will match exactly what `^` matches.
`+`	Matches the previous regexp any number of times, but at least once. For example, `ca+r` matches strings beginning with `c` and ending with `r`, with at least one `a` in-between. An empty regexp followed by `+` matches an empty part of the input.
`?`	Matches the previous regexp either 0 or 1 times. For example, `ca?r` matches either the string `cr` or `car`, and nothing else. An empty regexp followed by `?` matches an empty part of the input.
`^`	Matches the next regexp as long as it is at the beginning of a line. For example, `^foo` matches the string `foo` as long as it is at the beginning of a line.
`$`	Matches the previous regexp as long as it is at the end of a line. For example, `foo$` matches the string `foo` as long as it is at the end of a line.
`[ ]`	Contains a character set to be used for matching, where the other special characters mentioned do not apply. The empty string is automatically part of the character set. For example, `[a.b]` matches either `a` or `.` or `b` or the empty string. The regexp `c[ad]*r` matches strings beginning with `c` and ending with `r` , with any number of `a`s and `d`s in-between. The characters `-` and `^` have special meanings inside character sets. `-` defines a range and `^` defines a complement character set. For example, `[a-d]` matches any character in the range `a` to `d` inclusive, and `[^ab]` matches any character except `a` or `b`.
`\`	Quotes the special characters. For example, `\` matches the character `` (that is, `*` has lost its special meaning).
`\\|`	Specifies an alternative. For example, `ab\\|cd` matches either `ab` or `cd`.
`$`, `$`	Provides a grouping construct. For example, `ab$cd\\|ef$` matches either `abcd` or `abef`.