Regular Expressions/POSIX Basic Regular Expressions
The POSIX Basic Regular Expression (BRE) syntax provided extensions to achieve consistency between utility programs such as grep, sed and awk. These extensions are not supported by some traditional implementations of Unix tools.
Traditional Unix regular expression syntax followed common conventions that often differed from tool to tool. The POSIX Basic Regular Expressions syntax was developed by the IEEE, together with an extended variant called Extended Regular Expression syntax. These standards were designed mostly to provide backward compatibility with the traditional Simple Regular Expressions syntax, providing a common standard which has since been adopted as the default syntax of many Unix regular expression tools.
In POSIX Basic Regular Expression syntax, most characters are treated as literals — they match only themselves (e.g.,
a matches "a"). The exceptions, listed below, are called metacharacters or metasequences.
||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 bracket expression. Matches a single character that is contained within the brackets. For example,
||Matches a single character that is not contained within the brackets. For example,
||Matches the starting position within the string, if it is the first character of the regular expression.|
||Matches the ending position of the string, if it is the last character of the regular expression.|
||Matches the preceding element zero or more times. For example,
|Matches the preceding element exactly m times. For example,
|Matches the preceding element at least m times. For example,
|Matches the preceding element at least m and not more than n times. For example,
|Defines a subexpression. It is treated as a single element. For example,
||Matches what the nth marked subexpression matched, where n is a digit from 1 to 9. This construct is theoretically irregular (an expression with this construct does not obey the mathematical definition of regular expression), and was not adopted in the POSIX ERE syntax.|
.atmatches any three-character string ending with "at", including "hat", "cat", and "bat".
[hc]atmatches "hat" and "cat".
[^b]atmatches all strings matched by
^[hc]atmatches "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.
\[.\]matches any single character surrounded by "[" and "]" since the brackets are escaped, for example: "[a]" and "[b]".
The POSIX standard defines some classes or categories of characters as shown below. These classes are used within brackets.
|POSIX class||similar to||meaning|
|[:alpha:]||[A-Za-z]||upper- and lowercase letters|
|[:alnum:]||[A-Za-z0-9]||digits, upper- and lowercase letters|
|[:punct:]||punctuation (all graphic characters except letters and digits)|
|[:blank:]||[ \t]||space and TAB characters only|
|[:space:]||[ \t\n\r\f\v]||blank (whitespace) characters|
|[:graph:]||[^ [:cntrl:]]||graphic characters (all characters which have graphic representation)|
|[:print:]||[[:graph] ]||graphic characters and space|
a[[:digit:]]bmatches "a0b", "a1b", ..., "a9b".
a[:digit:]bis an error: character classes must be in brackets
[[:digit:]abc]matches any digit, "a", "b", and "c".
[abc[:digit:]]is the same as above
[^ABZ[:lower:]]matches any character except lowercase letters, A, B, and Z.
Collating symbols, like character classes, are used in brackets and have the form
ch is a digraph. Collating systems are defined by the locale.
Equivalence classes, like character classes and collating symbols, are used in brackets and have the form
[=a=]. They stand for any character which is equivalent to the given. According to the standard,
For example, if 'a', 'à', and 'â' belong to the same equivalence class, then "[[=a=]b]", "[[=à=]b]", and "[[=â=]b]" are each equivalent to "[aàâb]".
Equivalence classes, like collating symbols, are defined by the locale.
Use in Tools
Tools and languages that utilize this regular expression syntax include: