PostgreSQL: Documentation: Manuals: PostgreSQL 7.3: Pattern Matching: "6.6.2. SIMILAR TO and SQL99 Regular Expressions

string SIMILAR TO pattern [ESCAPE escape-character]
string NOT SIMILAR TO pattern [ESCAPE escape-character]

The SIMILAR TO operator returns true or false depending on whether its pattern matches the given string. It is much like LIKE, except that it interprets the pattern using SQL99's definition of a regular expression. SQL99's regular expressions are a curious cross between LIKE notation and common regular expression notation.

Like LIKE, the SIMILAR TO operator succeeds only if its pattern matches the entire string; this is unlike common regular expression practice, wherein the pattern may match any part of the string. Also like LIKE, SIMILAR TO uses % and _ as wildcard characters denoting any string and any single character, respectively (these are comparable to .* and . in POSIX regular expressions).

In addition to these facilities borrowed from LIKE, SIMILAR TO supports these pattern-matching metacharacters borrowed from POSIX regular expressions:

*

| denotes alternation (either of two alternatives).
*

* denotes repetition of the previous item zero or more times.
*

+ denotes repetition of the previous item one or more times.
*

"