Regular Expressions/POSIX Extended Regular Expressions

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The more modern "extended" regular expressions can often be used with modern Unix utilities by including the command line flag "-E".

POSIX extended regular expressions are similar in syntax to the traditional Unix regular expressions, with some exceptions. The following metacharacters are added:

Metacharacter Description
. Matches any single character (many applications exclude newlines, and exactly which characters are considered newlines is flavor, character encoding, and platform specific, but it is safe to assume that the line feed character is included). Within POSIX bracket expressions, the dot character matches a literal dot. For example, a.c matches "abc", etc., but [a.c] matches only "a", ".", or "c".
[ ] A bracket expression. Matches a single character that is contained within the brackets. For example, [abc] matches "a", "b", or "c". [a-z] specifies a range which matches any lowercase letter from "a" to "z". These forms can be mixed: [abcx-z] matches "a", "b", "c", "x", "y", or "z", as does [a-cx-z].

The - character is treated as a literal character if it is the last or the first (after the ^) character within the brackets: [abc-], [-abc]. Note that backslash escapes are not allowed. The ] character can be included in a bracket expression if it is the first (after the ^) character: []abc].

[^ ] Matches a single character that is not contained within the brackets. For example, [^abc] matches any character other than "a", "b", or "c". [^a-z] matches any single character that is not a lowercase letter from "a" to "z". As above, literal characters and ranges can be mixed.
^ Matches the starting position within the string. In line-based tools, it matches the starting position of any line.
$ Matches the ending position of the string or the position just before a string-ending newline. In line-based tools, it matches the ending position of any line.
BRE: \( \)
ERE: ( )
Defines a marked subexpression. The string matched within the parentheses can be recalled later (see the next entry, \n). A marked subexpression is also called a block or capturing group.
\n Matches what the nth marked subexpression matched, where n is a digit from 1 to 9. This construct is theoretically irregular and was not adopted in the POSIX ERE syntax. Some tools allow referencing more than nine capturing groups.
* Matches the preceding element zero or more times. For example, ab*c matches "ac", "abc", "abbbc", etc. [xyz]* matches "", "x", "y", "z", "zx", "zyx", "xyzzy", and so on. \(ab\)* matches "", "ab", "abab", "ababab", and so on.
BRE: \{m,n\}
ERE: {m,n}
Matches the preceding element at least m and not more than n times. For example, a\{3,5\} matches only "aaa", "aaaa", and "aaaaa". This is not found in a few older instances of regular expressions.
  • + — Match the last "block" one or more times - "ba+" matches "ba", "baa", "baaa" and so on
  • ? — Match the last "block" zero or one times - "ba?" matches "b" or "ba"
  • | — The choice (or set union) operator: match either the expression before or the expression after the operator - "abc|def" matches "abc" or "def".

Also, backslashes are removed: \{...\} becomes {...} and \(...\) becomes (...). Examples:

  • "[hc]+at" matches with "hat", "cat", "hhat", "chat", "hcat", "ccchat" etc.
  • "[hc]?at" matches "hat", "cat" and "at"
  • "([cC]at)|([dD]og)" matches "cat", "Cat", "dog" and "Dog"

The characters (,),[,],.,*,?,+,^, and $ are special symbols and have to be escaped with a backslash symbol in order to be treated as literal characters. For example:

"a\.(\(|\))" matches with the string "a.)" or "a.("

Modern regular expression tools allow a quantifier to be specified as non-greedy, by putting a question mark after the quantifier: (\[\[.*?\]\]).

Character classes[edit]

The POSIX standard defines some classes or categories of characters as shown in the following table:

POSIX class similar to meaning
[:upper:] [A-Z] uppercase letters
[:lower:] [a-z] lowercase letters
[:alpha:] [A-Za-z] upper- and lowercase letters
[:alnum:] [A-Za-z0-9] digits, upper- and lowercase letters
[:digit:] [0-9] digits
[:xdigit:] [0-9A-Fa-f] hexadecimal digits
[:punct:] [.,!?:...] punctuation
[:blank:] [ \t] space and TAB characters only
[:space:] [ \t\n\r\f\v] blank (whitespace) characters
[:cntrl:] control characters
[:graph:] [^ \t\n\r\f\v] printed characters
[:print:] [^\t\n\r\f\v] printed characters and space

Links:


Use in Tools[edit]

Tools and languages that utilize this regular expression syntax include:

  • AWK - uses a superset of the extended regular expression syntax

Links[edit]