Getting Started with Regular Expressions

In computer science, a regular expression is a single string that describes or matches a sequence of strings conforming to a particular syntactic rule. In many text editors or other tools, regular expressions are commonly used to search for and/or replace text content that matches a specific pattern. Numerous programming languages support string manipulation using regular expressions. For example, Perl has a powerful regular expression engine built in. The concept of regular expressions was originally popularized by Unix utilities such as sed and grep. Regular expressions are often abbreviated as "regex", with singular forms like regexp or regex, and plural forms such as regexps, regexes, or regexen.

A regular expression is essentially a formula for matching a class of strings based on a certain pattern. Many people avoid using them because they appear strange and complex—unfortunately, this article won't change that impression. However, with a bit of practice, you may find that writing these seemingly complicated expressions is actually quite straightforward. Once you understand them, you can reduce hours of tedious and error-prone text processing tasks to just minutes—or even seconds. Regular expressions are widely supported across various text editing software, libraries (such as Rogue Wave's tools.h++), and scripting tools (like awk, grep, and sed). Even interactive IDEs such as Microsoft's Visual C++ now offer support for regular expressions.

In the following sections, we will explain the usage of regular expressions through examples. Most examples are based on the text replacement command in vi and the file search command in grep. However, these are representative cases, and the concepts discussed can be applied to sed, awk, perl, and other programming languages that support regular expressions. You may refer to the section on regular expressions in different tools for additional examples of their usage. A brief explanation of the text substitution command (s) in vi is also included at the end for reference.