(direct link) Lookbehind: Fixed-Width / Constrained Width / Infinite Width One important difference is whether lookbehind accepts variable-width patterns. That is, it allows to match a pattern only if there’s something before it. Positive lookbehind is used to select a pattern in a string when another known pattern is just behind it. Philip Hazel started writing PCRE in summer 1997.PCRE's syntax is much more powerful and flexible than either of the POSIX regular expression flavors (BRE, ERE) and than that of many other regular-expression libraries. 2 Solutions. In these cases, you can optimize regular expression performance by removing the nested quantifiers and replacing the outer subexpression with a zero-width lookahead or lookbehind assertion. Maybe you should ask this on Stack Overflow ? subexpression) Zero-width negative lookbehind. Mov file size very small compared to pngs. If the regex inside the branch reset group has multiple alternatives with capturing groups, then the capturing group numbers are the same in all the alternatives. Positive Lookbehind. Perl Compatible Regular Expressions (PCRE) is a library written in C, which implements a regular expression engine, inspired by the capabilities of the Perl programming language. Javascript lookbehind alternative in regex with both lookbehind and lookahead. The regex inside the parenthesis is matched as usuall, however after matching the regex engine looks if a certain word follows it or precedes it or follows and precedes it. your coworkers to find and share information. But the digit shouldn't be part of the match. Maybe you should ask this on Stack Overflow ? Alternatives are tried from left to right, so the first alternative found for which the entire expression matches, is the one that is chosen. in the assertion does not match the whole surrogate pair that represents the character U+1D11E; the lookbehind (?<=a.) Javascript doesn't have regex lookbehind. A lookbehind assertion is needed to achieve this effect. Negative lookbehind: (?