Structure of Regular Expressions

The table below describes the structure of regular expressions and shows which characters may be used:

Character Description
[1] char Matches itself, unless it is a special character (metachar): . \ [ ] * + ^ $
[2] . Matches any character
[3] \

Matches the subsequent character unless it is a left or right round bracket, a digit 1 to 9 or a left or right angle bracket (see [7], [8] and [9]).

It is used as an escape character for all other meta-characters, and itself. When used in a set (see [4]), it is treated as an ordinary character.

[4] [set]

Matches one of the characters in the set. If the first character in the set is "^", it matches a character NOT in the set. The shorthand “S-E” is used to specify a set of characters from S to E, inclusively. The special characters "]" and "-" have no special meaning if they appear as the first chars in the set.

Examples:

[a-z] any lowercase letter

[^]-] any character except ] and -

[^A-Z] any character except uppercase letters

[a-zA-Z] any letter

[5] * Any regular expression of [1] to [4] followed by the closure character (*) matches a string of zero or more characters that have that form.
[6] + Same as [5], except it matches one or more.
[7] A regular expression of form [1] to [10] enclosed as \(form\) matches whatever the specified form matches. The enclosure creates a set of tags, used for [8] and for pattern substitution. The tagged forms are numbered starting from 1.
[8] A \ character followed by a digit 1 to 9 matches whatever a previously tagged regular expression ([7]) matched. For example, “\5” represents the fifth pattern specified in the “\(form\)” format.
[9]

\<

\>

A regular expression starting with a \< construct and/or ending with a \> construct, restricts the pattern matching to the beginning of a word, and/or the end of a word. A word is defined to be a character string beginning and/or ending with the characters A-Z a-z 0-9 and _. It must also be preceded and/or followed by any character other than those mentioned.
[10] A composite regular expression xy, where x and y are of form [1] to [10] matches the longest match of x followed by a match for y.
[11] ^ $ A regular expression starting with a ^ character and/or ending with a $ character restricts the pattern matching to the beginning of the line or to the end of line. Elsewhere in the expression, ^ and $ are treated as ordinary characters.

Example

Pattern Matches
foo*.* fo foo fooo foobar fobar foxx ...
fo[ob]a[rz] fobar fooar fobaz fooaz
foo\\+ foo\ foo\\ foo\\\ ...
\(foo\)[1-3]\1(same as foo[1-3]foo) foo1foo foo2foo foo3foo
\(fo.*\)-\1 foo-foo fo-fo fob-fob foobar-foobar ...