Javascript Regex: create new a lookbehind regular expression with variable?

Question

I want to create new a RegExp with variable by JavaScript. But I got trouble when I want to create new a lookaround RegExp:

var xvar = "_on";
var regex = new RegExp("(?<!" + xvar + ")(.gif|.jpg|.png)$");
#=> SyntaxError: Invalid regular expression: /(?<!_on)(.gif|.jpg|.png)$/: Invalid group

I tried to escape all special characters follow Creating regexp with special characters, then it can create a new RegExp but just like a string, not a lookaround regex.

/\(\?<!_on\)\(\.gif\|\.jpg\|\.png\)\$/

Can anyone help me?

one way is to reverse the string... depends on what you are doing. — d'alar'cop, Aug 26 '13 at 07:01
What's the ultimate goal? Why can't you use a normal, non-capturing group, as in `"(?:" + xvar + ")(.gif|.jpg|.png)$"`? — Alan Moore, Aug 26 '13 at 08:44

tokkonopapa · Answer 1 · 2013-08-26T15:31:19.770

-2

I don't know your goal of RegExp but quantifier (...) causes error in this situation. For example, use non-capturing parentheses (?:...).

var regex = new RegExp("(?:<!" + xvar + ")(?:.gif|.jpg|.png)$");

Refer to: Regular Expressions - JavaScript | MDN.

@Alan Moore: you're right.

edited Aug 26 '13 at 15:31

answered Aug 26 '13 at 08:29

tokkonopapa

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He's not trying to match `<!`, he's trying to use a negative lookbehind, which is not supported in JavaScript regexes. – Alan Moore Aug 26 '13 at 08:46

Javascript Regex: create new a lookbehind regular expression with variable?

1 Answers1