Download - The Bastards Book of Regular Expressions

Capture, Reuse 81Parentheses for captured groupsParentheses have what might be called a major side effect regarding the patterns they group together.When part of a regex is inside a parentheses, the regex engine contains a backreference to thepattern captured inside the parentheses.Let’s go back to the last-name-first-name example at the beginning of the chapter:Smith, ShellyPerry, DwayneWhat’s the pattern?We want to capture the words on both sides of the comma.This is easy enough; the regex pattern is:(\w+), (\w+)Ah, but what do we replace it with?Backreference notationUp until now, the Replace part of the Find-and-Replace action hasn’t involved many specialcharacters. In fact, nearly none of the special characters in regex-land, such as +, [ ], ˆ, and soforth, have any meaning beyond their literal values.But the backslash still wields its escaping power in the Replace field.Backreference notationInside the Replace field, using a backslash followed by a number, will denote thevalue inside the corresponding captured group. In other words, \1 contains thevalue of the first captured group. And \2 contains the value for the second group,and so forth.Let’s consider this simple use of parentheses in a pattern:(ab)(cd)(e)If the Replace field is set to:.\1\2\3