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13.2.2 Characters and character classes

Typically a character in the regexp matches the same character in the text string. Sometimes it is necessary or convenient to use a regexp metasequence to refer to a single character. Thus, metasequences \n, \r, \t, and \. match the newline, return, tab and period characters respectively.

The metacharacter period (.) matches any character other than newline.

(pregexp-match "p.t" "pet") ⇒ ("pet")

It also matches pat, pit, pot, put, and p8t but not peat or pfffft.

A character class matches any one character from a set of characters. A typical format for this is the bracketed character class [...], which matches any one character from the non-empty sequence of characters enclosed within the brackets.(5) Thus "p[aeiou]t" matches pat, pet, pit, pot, put and nothing else.

Inside the brackets, a hyphen (-) between two characters specifies the ascii range between the characters. Eg, "ta[b-dgn-p]" matches tab, tac, tad, and tag, and tan, tao, tap.

An initial caret (^) after the left bracket inverts the set specified by the rest of the contents, ie, it specifies the set of characters other than those identified in the brackets. Eg, "do[^g]" matches all three-character sequences starting with do except dog.

Note that the metacharacter ^ inside brackets means something quite different from what it means outside. Most other metacharacters (., *, +, ?, etc) cease to be metacharacters when inside brackets, although you may still escape them for peace of mind. - is a metacharacter only when it’s inside brackets, and neither the first nor the last character.

Bracketed character classes cannot contain other bracketed character classes (although they contain certain other types of character classes — see below). Thus a left bracket ([) inside a bracketed character class doesn’t have to be a metacharacter; it can stand for itself. Eg, "[a[b]" matches a, [, and b.

Furthermore, since empty bracketed character classes are disallowed, a right bracket (]) immediately occurring after the opening left bracket also doesn’t need to be a metacharacter. Eg, "[]ab]" matches ], a, and b.


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