javascript regex - look behind alternative

Question

Here is a regex that works fine in most regex implementations      lt  filename   js    This matches  js for a string which ends with  js except for filename js  Javascript doesn t have regex lookbehind  Is anyone able put together an alternative regex which achieve the same result and works in javascript   Here are some thoughts  but needs helper functions  I was hoping to achieve it just with a regex  http   blog stevenlevithan com archives mimic-lookbehind-javascript

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Let s suppose you want to find all int not preceded by unsigned   With support for negative look-behind      lt  unsigned  int   Without support for negative look-behind       unsigned    9     0 8  int   Basically idea is to grab n preceding characters and exclude match with negative look-ahead  but also match the cases where there s no preceeding n characters   where n is length of look-behind    So the regex in question      lt  filename   js    would translate to       filename   8     0 7    js    You might need to play with capturing groups to find exact spot of the string that interests you or you want t to replace specific part with something else

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If you can look ahead but back  you could reverse the string first and then do a lookahead  Some more work will need to be done  of course

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Below is a positive lookbehind JavaScript alternative showing how to capture the last name of people with  Michael  as their first name   1  Given this text   const exampleText    Michael  how are you  - Cool  how is John Williamns and Michael Jordan  I don t know but Michael Johnson is fine  Michael do you still score points with LeBron James  Michael Green Miller and Michael Wood      get an array of last names of people named Michael  The result should be    Jordan   Johnson   Green   Wood    2  Solution   function getMichaelLastName2 text      return text      match     Michael    A-Z  a-z    g       map person   gt  person slice person indexOf      1          or even      map person   gt  person slice 8       since we know the length of  Michael     3  Check solution  console log JSON stringify     getMichaelLastName exampleText              Jordan   Johnson   Green   Wood     Demo here  http   codepen io PiotrBerebecki pen GjwRoo  You can also try it out by running the snippet below    x000D   x000D  const inputText    Michael  how are you  - Cool  how is John Williamns and Michael Jordan  I don t know but Michael Johnson is fine  Michael do you still score points with LeBron James  Michael Green Miller and Michael Wood    x000D   x000D   x000D   x000D  function getMichaelLastName text    x000D    return text x000D       match     Michael    A-Z  a-z    g  x000D       map person   gt  person slice 8    x000D    x000D   x000D  console log JSON stringify     getMichaelLastName inputText         x000D   x000D   x000D

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EDIT  From ECMAScript 2018 onwards  lookbehind assertions  even unbounded  are supported natively   In previous versions  you can do this          filename  js       js    This does explicitly what the lookbehind expression is doing implicitly  check each character of the string if the lookbehind expression plus the regex after it will not match  and only then allow that character to match                       Start of string                     Try to match the following                      First assert that we can t match the following    filename  js      filename js                      and end-of-string                     End of negative lookahead                     Match any character                     Repeat as needed   js                Match  js                     End of string   Another edit   It pains me to say  especially since this answer has been upvoted so much  that there is a far easier way to accomplish this goal  There is no need to check the lookahead at every character         filename  js      js    works just as well                       Start of string                     Assert that we can t match the following                      any string     filename  js      followed by filename js                     and end-of-string                     End of negative lookahead                     Match any string   js                Match  js                     End of string

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This is an equivalent solution to Tim Pietzcker s answer  see also comments of same answer          filename  js      js    It means  match   js except  filename js   To get to this solution  you can check which patterns the negative lookbehind excludes  and then exclude exactly these patterns with a negative lookahead

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filename     js works for me  tested against    test js match blabla js match filename js no match   A proper explanation for this regex can be found at Regular expression to match string not containing a word   Look ahead is available since version 1 5 of javascript and is supported by all major browsers  Updated to match filename2 js and 2filename js but not filename js       filename  js        js

[javascript] javascript regex - look behind alternative?

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