A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions by using various operators to combine smaller expressions.
The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any metacharacter with special meaning can be quoted by preceding it with a backslash.
A list of characters enclosed by [ and ] matches any single character in that list; if the first character of the list is the caret then it matches any character not in the list. For example, the regular expression [0123456789] matches any single digit. A range of characters can be specified by giving the first and last characters, separated by a hyphen. Finally, certain named classes of characters are predefined. Their names are self explanatory, and they are [:alnum:], [:alpha:], [:cntrl:], [:digit:], [:graph:], [:lower:], [:print:], [:punct:], [:space:], [:upper:], and [:xdigit:].
For example, [[:alnum:]] means [0-9A-Za-z], except the latter form depends upon the POSIX locale and the ASCII character encoding, whereas the former is independent of locale and character set. (Note that the brackets in these class names are part of the symbolic names and must be included in addition to the brackets delimiting the bracket list.) Most metacharacters lose their special meaning inside lists. To include a literal ] place it first in the list. Similarly, to include a literal ^ place it anywhere but first. Finally, to include a literal - place it last.
The period . matches any single character. The symbol w is a synonym for [[:alnum:]] and W is a synonym for [^[:alnum]].
The caret ^ and the dollar sign $ are metacharacters that respectively match the empty string at the beginning and end of a line. The symbols < and > respectively match the empty string at the beginning and end of a word. The symbol matches the empty string at the edge of a word, and B matches the empty string provided it's not at the edge of a word.
A regular expression may be followed by one of several repetition operators:
Table A.1. Repetition Operators
| Operator | Meaning |
|---|---|
?
| The preceding item is optional and matched at most once. |
*
| The preceding item will be matched zero or more times. |
+
| The preceding item will be matched one or more times. |
{n}
| The preceding item is matched exactly n times. |
{n,}
| The preceding item is matched n or more times. |
{n,m}
| The preceding item is matched at least n times, but not more than m times. |
Two regular expressions can be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated subexpressions.
Two regular expressions can be joined by the infix operator |; the resulting regular expression matches any string matching either subexpression.
Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole subexpression can be enclosed in parentheses to override these precedence rules.
The backreference
, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression.
In basic regular expressions the metacharacters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions ?, +, {, |, (, and ).