![]() ![]() This is explored on the section on possessive quantifiers. When a series of characters only makes sense as a block, using an atomic group can prevent needless backtracking. If, before the atomic group, there were other options to which the engine can backtrack (such as quantifiers or alternations), then the whole atomic group can be given up in one go. Because the group is atomic, it is unable to give up the second A, which would allow the rest of the pattern to match. After matching the AA in the atomic group, the engine tries to match the, succeeds by matching the C, then tries to match the token C but fails as the end of the string has been reached. This will fail against AAC, whereas (?:A+)C would have succeeded. ![]() B part of the alternation, which would also succeed, and allow the final token C to match. Because it is atomic, it is unable to try the. After matching the A in the atomic group, the engine tries to match the C but fails. This will fail against ABC, whereas (?:A|.B)C would have succeeded. An atomic group won't do that: it's all or nothing. Likewise, if the group contained an alternation, the engine would try the next branch. If the regex fails later down the string and needs to backtrack, a regular group containing a quantifier would give up characters one at a time, allowing the engine to try other matches. Now that these three "big ones" are out of the way, let's drill into the syntax.Īn atomic group is an expression that becomes solid as a block once the regex leaves the closing parenthesis. The second is a subroutine call that matches the sub-pattern contained within the capturing parentheses of Group 1. The first is a conditional expression that tests whether Group 1 has been captured. (? … ) must be a lookahead, right? Not so. (?: … ) contains a non-capturing group, while (?= … ) is a lookahead. These false twins have very different jobs. ✽ Pre-Defined Subroutines: (?(DEFINE)( … )( … )) and (?&foo)Ĭonfusing Couples Confusing Couple #1: (?: … ) and (?= … ) I'll start by pointing out three confusing couples details of usage will follow.įor easy navigation, here are some jumping points to various sections of the page: To facilitate study, I have pulled all the (? … ) usages I know about into one place. But (?: … ) looks a lot like (?= … ), so that at some point they are bound to clash in the mind of the regex apprentice. In the regex tutorials and books I have read, these various points of syntax are introduced in stages. ![]() One of the things that make regexes hard to read for beginners is that many points of syntax that serve vastly different purposes all start with the same two characters: The heroes who expanded regular expressions (such as Henry Spencer and Larry Wall) followed in these footsteps. Maybe they were into hieroglyphs, maybe they were into cryptography, or maybe that was just the way you did things when you only had a few kilobytes or RAM. Stephen Kleene and Ken Thompson, who started them, obviously wanted something very compact. I don't know the fine details of the history of regular expressions. I thought I would bring them all together in one place. A question mark inside a parenthesis: So many uses! ![]()
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