That's unfortunate, but I'm not content with just resigning to that fact. But the main issue that makes JavaScript regex so obnoxious is its lack of features. Other than those use cases, you can just as well make the assertion a real part of the regular expression. I am writing a code to parse my inbox mail body to organize the resulting variables as a spreadsheet row. In the Quick Reference pane (bottom right corner), it shows Negative Lookbehinds are supported for some browsers, but not all. For those interested, @alexchandel was saying that while V8 has added experimental lookbehind support, it is still inferior to many alternative regex libraries in his opinion: V8 still doesn't have have async callbacks, and catastrophic backtracking and misspelt regexes are the primary source of hangs in find-and-replace, for example #557 and #856.. And JavaScript does support lookaheads; it doesn't support lookbehinds though. Regex lookahead and lookbehind assertion with an example The expression that I used in the example is the following Sometimes we need to look if a string matches or contains a certain pattern and that's what regular expressions (regex) are for. But sometimes we have the condition that this pattern is preceded or followed by another certain pattern. Inside the lookahead, we have the trivial … In javascript the majority of browsers don't yet support lookbehinds. ES2018 continues the work of previous editions of ECMAScript by making regular expressions more useful. Assertions include boundaries, which indicate the beginnings and endings of lines and words, and other patterns indicating in some way that a match is possible (including look-ahead, look-behind, and conditional expressions). Suggestion. Checklist. Further in the pattern \1 means “find the same text as in the first group”, exactly the same quote in our case. New features include lookbehind assertion, named capture groups, s (dotAll) flag, and Unicode property escapes. Support ECMA 2018 javascript regex. Start by trying to identify why you want to do it. As in, 12 - 23 12 - -23 -12 - 23 -12 - -23 Please ignore the spaces. What I want is a regex valid in javascript that could mimic that behavior. Maybe you should ask this on Stack Overflow ? 8.10.0 (support for s (dotAll) flag and lookbehind assertions) 10.0.0 (full support) Wrapping Up. Lookbehind is part of the ECMAScript 2018 specification. Tag: javascript,regex. Unlike lookaheads, JavaScript doesn't support regex lookbehind syntax. The lack of generalized assertions, like the lack of backreferences, is not a statement on our part about regular expression style. Is there a workaround , or do I try a different solution? When considering using the user agent string to detect which browser is being used, your first step is to try to avoid it if possible. The syntax is: Positive lookbehind: (?<=Y)X, matches X, but only if there’s Y before it. He … lookbehind regex. Lookahead and lookbehind (commonly referred to as “lookaround”) are useful when we’d like to match something depending on the context before/after it. Thanks for your answer. Note the word "balll" -- true lookbehind should have suppressed the first 2 l's but matched the 2nd pair. But by matching the first 2 l's and then ignoring that match as a false positive, the regexp engine proceeds from the end of that match, and ignores any characters within the false positive. ... there is a (?