Within a character list, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, using the locale's collating sequence and character set. For example, in the default C locale, `[a-dx-z]' is equivalent to `[abcdxyz]'. Many locales sort characters in dictionary order, and in these locales, `[a-dx-z]' is typically not equivalent to `[abcdxyz]'; instead it might be equivalent to `[aBbCcDdxXyYz]', for example. To obtain the traditional interpretation of bracket expressions, you can use the C locale by setting the LC_ALL environment variable to the value `C'.
To include one of the characters `\', `]', `-', or `^' in a character list, put a `\' in front of it. For example:
[d\]]
matches either `d' or `]'.
This treatment of `\' in character lists is compatible with other awk implementations and is also mandated by POSIX. The regular expressions in awk are a superset of the POSIX specification for Extended Regular Expressions (EREs). POSIX EREs are based on the regular expressions accepted by the traditional egrep utility.
Character classes are a new feature introduced in the POSIX standard. A character class is a special notation for describing lists of characters that have a specific attribute, but the actual characters can vary from country to country and/or from character set to character set. For example, the notion of what is an alphabetic character differs between the United States and France.
A character class is only valid in a regexp inside the brackets of a character list. Character classes consist of `[:', a keyword denoting the class, and `:]'. table-char-classes lists the character classes defined by the POSIX standard.
| Class | Meaning
|
|---|---|
[:alnum:] | Alphanumeric characters.
|
[:alpha:] | Alphabetic characters.
|
[:blank:] | Space and TAB characters.
|
[:cntrl:] | Control characters.
|
[:digit:] | Numeric characters.
|
[:graph:] | Characters that are both printable and visible.
(A space is printable but not visible, whereas an `a' is both.)
|
[:lower:] | Lowercase alphabetic characters.
|
[:print:] | Printable characters (characters that are not control characters).
|
[:punct:] | Punctuation characters (characters that are not letters, digits,
control characters, or space characters).
|
[:space:] | Space characters (such as space, TAB, and formfeed, to name a few).
|
[:upper:] | Uppercase alphabetic characters.
|
[:xdigit:] | Characters that are hexadecimal digits.
|
Table 2.1: POSIX Character Classes
For example, before the POSIX standard, you had to write /[A-Za-z0-9]/
to match alphanumeric characters. If your
character set had other alphabetic characters in it, this would not
match them, and if your character set collated differently from
ASCII, this might not even match the ASCII alphanumeric characters.
With the POSIX character classes, you can write
/[[:alnum:]]/ to match the alphabetic
and numeric characters in your character set.
Two additional special sequences can appear in character lists. These apply to non-ASCII character sets, which can have single symbols (called collating elements) that are represented with more than one character. They can also have several characters that are equivalent for collating, or sorting, purposes. (For example, in French, a plain “e” and a grave-accented “è” are equivalent.) These sequences are:
[[.ch.]] is a regexp that matches this collating element, whereas
[ch] is a regexp that matches either `c' or `h'.
[[=e=]] is a regexp
that matches any of `e', `é', or `è'.
These features are very valuable in non-English-speaking locales.
Caution: The library functions that gawk uses for regular expression matching currently recognize only POSIX character classes; they do not recognize collating symbols or equivalence classes.