Page URL: https://www.regular-expressions.info/lookaround.html Page last updated: 09 March 2020 Site last updated: 05 October 2020 Copyright © 2003-2021 Jan Goyvaerts. First the lookaround captures 123 into \1. is valid because it … (The only exception is Tcl, which treats all groups inside lookahead as non-capturing.) Keep this in mind. The string literal "\b", for example, matches a single backspace character when interpreted as a regular expression, while "\\b" matches a … The engine again steps back one character, and notices that the a can be matched there. In some situations you may get an error. You can use literal text, character escapes, Unicode escapes other than \X, and character classes. These bugs were fixed in Java 6. REGEX_1 makes sure that the length of string is indeed 4, but doesn't consume any characters so that search for REGEX_2 starts at the same location. The syntax is: Positive lookbehind: (?<=Y)X, matches X, but only if there’s Y before it. This repeated stepping back through the subject string kills performance when the number of possible lengths of the lookbehind grows. Bør det ikke kasseres? The lookahead was successful, so the engine continues with i. The string containing regular expression must be compiled to the instance of the Pattern class. Java applies lookbehind by going through the target string from right to left, while going through the regex from left to right. When evaluating the lookbehind, the regex engine determines the length of the regex inside the lookbehind, steps back that many characters in the subject string, and then applies the regex inside the lookbehind from left to right just as it would with a normal regex. Some regex flavors (Perl, PCRE, Oniguruma, Boost) only support fixed-length lookbehinds, but offer the \K feature, which can be used to simulate variable-length lookbehind at the start of a pattern. If you want to find a word not ending with an “s”, you could use \b\w+(?