Rx

3.1: An Introduction to Regexps

In the simplest cases, a regexp is just a literal string that must match exactly. For example, the pattern:

regexp

matches the string "regexp" and no others.

Some characters have a special meaning when they occur in a regexp. They aren't matched literally as in the previous example, but instead denote a more general pattern. For example, the character * is used to indicate that the preceeding element of a regexp may be repeated 0, 1, or more times. In the pattern:

smooo*th

the * indicates that the preceeding o can be repeated 0 or more times. So the pattern matches:

smooth
smoooth
smooooth
smoooooth
...

Suppose you want to write a pattern that literally matches a special character like * -- in other words, you don't want to * to indicate a permissible repetition, but to match * literally. This is accomplished by quoting the special character with a backslash. The pattern:

smoo\*th

matches the string:

smoo*th

and no other strings.

In seven cases, the pattern is reversed -- a backslash makes the character special instead of making a special character normal. The characters +, ?, |, (, and ) are normal but the sequences \+, \?, \|, \(, \), \{, and \} are special (their meaning is described later).

The remaining sections of this chapter introduce and explain the various special characters that can occur in regexps.