Regular Expressions Cookbook

Searching and Replacing with Regular Expressions

Search-and-replace is a common job for regular expressions. A search-and-replace function takes a subject string, a regular expression, and a replacement string as input. The output is the subject string with all matches of the regular expression replaced with the replacement text.

Although the replacement text is not a regular expression at all, you can use certain special syntax to build dynamic replacement texts. All flavors let you reinsert the text matched by the regular expression or a capturing group into the replacement. Recipes 2.20 and 2.21 explain this. Some flavors also support inserting matched context into the replacement text, as Recipe 2.22 shows. In Chapter 3, Recipe 3.16 teaches you how to generate a different replacement text for each match in code.

Many Flavors of Replacement Text

Different ideas by different regular expression software developers have led to a wide range of regular expression flavors, each with different syntax and feature sets. The story for the replacement text is no different. In fact, there are even more replacement text flavors than regular expression flavors. Building a regular expression engine is difficult. Most programmers prefer to reuse an existing one, and bolting a search-and-replace function onto an existing regular expression engine is quite easy. The result is that there are many replacement text flavors for regular expression libraries that do not have built-in search-and-replace ...