Question

I have a regular expression to match usernames (which functions in PHP using preg_match):

/[a-z]+(?(?=\-)[a-z]+|)\.[1-9][0-9]*/

This pattern matches usernames of the form abc.124, abc-abc.123, etc.

However, when I take this to JavaScript:

var re = new RegExp("/[a-z]+(?(?=\-)[a-z]+|)\.[1-9][0-9]*/"); 

I receive a syntax error:

SyntaxError: Invalid regular expression: /[a-z]+(?(?=-)[a-z]+|).[1-9][0-9]*/: Invalid group

The (?(?=\-)[a-z]+|) is to say if after [a-z]+ we see a - then assert that [a-z]+ is after it otherwise, match nothing. This all works great in PHP, but what am I missing about JavaScript that is different?

EDIT: I appreciate the comments, and now I have one last question regarding this:

    var str="accouts pending removal shen.1206";
    var patt= new RegExp("/[a-z]+(?:-[a-z]+)?\.[1-9][0-9]*/"); 
    var result=patt.exec(str);
    alert(result); 

This alert comes up as null? But if I do the following it works:

var patt=/[a-z]+(?:-[a-z]+)?\.[1-9][0-9]*/;
var result=patt.exec(str);
alert(result); 

Why does “new RegExp()” not work?

Answer 1

Different regular expression engines support different features. Conditionals are not supported by Javascript.

In any event, the conditional is unnecessary for your pattern. I would simplify your expression to /[a-z]+(?:-[a-z]+)?\.[1-9][0-9]*/, which is easier to understand and will work in both PHP’s PCRE and in Javascript.