13.2.4 POSIX character classes

A POSIX character class is a special metasequence of the form [:...:] that can be used only inside a bracketed expression. The POSIX classes supported are

[:alnum:]  letters and digits 
[:alpha:]  letters  
[:algor:]  the letters c, h, a and d 
[:ascii:]  7-bit ascii characters 
[:blank:]  widthful whitespace, ie, space and tab 
[:cntrl:]  ``control'' characters, viz, those with code < 32 
[:digit:]  digits, same as \d 
[:graph:]  characters that use ink 
[:lower:]  lower-case letters 
[:print:]  ink-users plus widthful whitespace  
[:space:]  whitespace, same as \s 
[:upper:]  upper-case letters 
[:word:]   letters, digits, and underscore, same as \w 
[:xdigit:] hex digits 

For example, the regexp "[[:alpha:]_]" matches a letter or underscore.

(pregexp-match "[[:alpha:]_]" "--x--") ⇒ ("x")
(pregexp-match "[[:alpha:]_]" "--_--") ⇒ ("_")
(pregexp-match "[[:alpha:]_]" "--:--") ⇒ #f

The POSIX class notation is valid only inside a bracketed expression. For instance, [:alpha:], when not inside a bracketed expression, will not be read as the letter class. Rather it is (from previous principles) the character class containing the characters :, a, l, p, h.

(pregexp-match "[[:alpha:]]" "--a--") ⇒ ("a")
(pregexp-match "[[:alpha:]]" "--_--") ⇒ #f

By placing a caret (^) immediately after [:, you get the inversion of that POSIX character class. Thus, [:^alpha] is the class containing all characters except the letters.

This document was generated on March 31, 2014 using texi2html 5.0.