lookbehind.js

> /(aaa)bbb/.exec("aaabbbccc") // lookbehind?
["aaabbb", "aaa"]
> /([^a]{3})bbb/.exec("dddbbbeee") // negative lookbehind?
["dddbbb", "ddd"]

lookbehind_replace.js

// Negative lookbehind regex in Javascript
// http://stackoverflow.com/questions/641407/javascript-negative-lookbehind-equivalent?answertab=votes#answer-641432
newString = string.replace(/([abcdefg])?m/, function($0,$1){ return $1?$0:'m';});

// examples of lookbehind
"Lookahead".replace(/(Look?)ahead/, function($0,$1){ return $1?$1+"behind":$0;})
> Lookbehind

"Lookahead".replace(/(See?)ahead/, function($0,$1){ return $1?$1+"behind":$0;})
> Lookahead

"Lookahead".replace(/([^Look])?ahead/, function($0,$1){ return $1?$0:'behind';})
> Lookbehind

prototype.js

String.prototype.lookbehind = function(p) {
  var s = new RegExp(p.replace(/\(\?<([=!])(.+)\)(.+)/,function($0,$1,$2,$3){return ($1=="!"?"[^"+$2+"]":$2)+"(" + $3 + ")";})).exec(this);
  return s?(s[1]?s[1]:s[0]):null;
};

"123".lookbehind("(?<=1)2"); 
> 2
"123".lookbehind("(?<!1)2"); 
> null