The term context refers to the subject text that the regular expression was applied to. There are three pieces of context: the subject text before the regex match, the subject text after the regex match, and the whole subject text. The text before the match is sometimes called the left context, and the text after the match is correspondingly the right context. The whole subject text is the left context, the match, and the right context.

.NET and Perl support «$`», «$'», and «$_» to insert all three forms of context into the replacement text. Actually, in Perl these are variables set after a successful regex match and are available in any code until the next match attempt. Dollar backtick is the left context. You can type the backtick on a U.S. keyboard by pressing the key to the left of the 1 key in the top-left corner of your keyboard. Dollar straight quote is the right context. The straight quote is the usual single quote. On a U.S. keyboard, it sits between the semicolon and Enter keys. Dollar underscore is the whole subject text. Like .NET and Perl, JavaScript uses «$`» and «$'» for left and right context. However, JavaScript does not have a token for inserting the entire subject text. You can recompose the subject text by inserting the whole regex match with «$&» between the left and right context.

Ruby supports left and right context via «\`» and «\'», and uses «\&» to insert the whole regex match. Like JavaScript, there is no token for the whole subject text.

Search and Replace with Regular Expressions in Chapter 1 describes the various replacement text flavors.

Recipe 3.15 explains how to use replacement text in source code.