Bracket
expressions are a list of characters and/or character classes enclosed
in brackets []. Use bracket expressions to match single characters
in a list, or a range of characters in a list. If the first character
of the list is the carat ^ then it matches characters that are not
in the list.
For example:
| Expression | Matches |
| [abc] | a, b, or c |
| [a-z] | a through z |
| [^abc] | Any character except a, b, or c |
| [[:alpha:]] | Any alphabetic character (see below) |
Each character class designates a set of characters
equivalent to the corresponding standard C isXXX function. For example,
[:alpha:] designates those characters for which isalpha() returns
true (example: any alphabetic character). Character classes must
be within bracket expression.
| Character class | Description |
| [:alpha:] | Alphabetic characters |
| [:digit:] | Digits |
| [:alnum:] | Alphabetic characters and numeric characters |
| [:cntrl:] | Control character |
| [:blank:] | Space and tab |
| [:space:] | All white space characters |
| [:graph:] | Non-blank (not spaces, control characters, or the like) |
| [:print:] | Like [:graph:], but includes the space character |
| [:punct:] | Punctuation characters |
| [:lower:] | Lowercase alphabetic |
| [:upper:] | Uppercase alphabetic |
| [:xdigit:] | Digits allowed in a hexadecimal number (0-9a-fA-F) |
For a case-insensitive expression, [:lower:]
and [:upper:] are equivalent to [:alpha:].