Regex

Traditional Unix regular expression syntax followed common conventions but often differed from tool to tool. The IEEE POSIX Basic Regular Expressions (BRE) standard (released alongside an alternative flavor called Extended Regular Expressions or ERE) was designed mostly for backward compatibility with the traditional syntax but provided a common standard which has since been adopted as the default syntax of many Unix regular expression tools, though there is often some variation or additional features. Many such tools also provide support for ERE syntax with command line arguments.

In the BRE syntax, most characters are treated as literals they match only themselves (i.e., a matches "a"). The exceptions, listed below, are called metacharacters or metasequences.

Quantification A quantifier after a token (such as a character) or group specifies how often that preceding element is allowed to occur. The most common quantifiers are ?, *, and +.
? The question mark indicates there is zero or one of the preceding element. For example, colou?r matches both "color" and "colour".
* The asterisk indicates there are zero or more of the preceding element. For example, ab*c matches "ac", "abc", "abbc", "abbbc", and so on.
+ The plus sign indicates that there is one or more of the preceding element. For example, ab+c matches "abc", "abbc", "abbbc", and so on, but not "ac".
Grouping Parentheses are used to define the scope and precedence of the operators (among other uses). For example, gray|grey and gr(a|e)y are equivalent patterns which both describe the set of "gray" and "grey".
| The choice (aka alternation or set union) operator matches either the expression before or the expression after the operator. For example, abc|def matches "abc" or "def".
. 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", and "z", as does [a-cx-z].

The - character is treated as a literal character if it is the last or the first character within the brackets, or if it is escaped with a backslash: [abc-], [-abc], or [a-bc].

[^ ] 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.
( ) 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.
{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.
{m,} Minumum occurance of element at least m times. For example, (a){3,} matches "aaa", "aaaa", and "aaaaa" etc. but not "a" and "aa".
{,n} Maximum occurance of element max. n times. For example, (a){,3} matches "a", "aa", and "aaa" but not "aaaa" etc..
{n} Exact occurance of element n times. For example, (a){3} matches "aaa".

Examples:

  • .at matches any three-character string ending with "at", including "hat", "cat", and "bat".
  • [hc]at matches "hat" and "cat".
  • [^b]at matches all strings matched by .at except "bat".
  • ^[hc]at matches "hat" and "cat", but only at the beginning of the string or line.
  • [hc]at$ matches "hat" and "cat", but only at the end of the string or line.
ASCII Description
[A-Za-z0-9] Alphanumeric characters
[A-Za-z] Alphabetic characters
[ t] Space and tab
[x00-x1Fx7F] Control characters
[0-9] Digits
[x21-x7E] Visible characters
[a-z] Lowercase letters
[x20-x7E] Visible characters and spaces
[-!"#$%&'()*+,./:;<=>?@[]_`{|}~] Punctuation characters
[ trnvf] Whitespace characters
[A-Z] Uppercase letters
[A-Fa-f0-9] Hexadecimal digits
([0-9]{4})-([0-9]{1,2})-([0-9]{1,2}) Date in ISO format (YYYY-MM-DD)