Learning Perl, Second Edition

7.3. Patterns

A regular expression is a pattern. Some parts of the pattern match single characters in the string of a particular type. Other parts of the pattern match multiple characters. First, we'll visit the single-character patterns and then the multiple-character patterns.

7.3.1. Single-Character Patterns

The simplest and most common pattern-matching character in regular expressions is a single character that matches itself. In other words, putting a letter a in a regular expression requires a corresponding letter a in the string.

The next most common pattern matching character is the dot ".". This matches any single character except newline (\n). For example, the pattern /a./ matches any two-letter sequence that starts with a and is not "a\n".

A pattern-matching character class is represented by a pair of open and close square brackets and a list of characters between the brackets. One and only one of these characters must be present at the corresponding part of the string for the pattern to match. For example,

/[abcde]/

matches a string containing any one of the first five letters of the lowercase alphabet, while

/[aeiouAEIOU]/

matches any of the five vowels in either lower- or uppercase. If you want to put a right bracket (]) in the list, put a backslash in front of it, or put it as the first character within the list. Ranges of characters (like a through z) can be abbreviated by showing the end points of the range separated by a dash (-); to get a literal dash in ...