regExpression.test

var regExpression = /^([a-zA-Z0-9_\-\.]+)$/; //line 2
//// var regExpression = "/" + "^([a-zA-Z0-9_\-\.]+)$" + "/"; //line 3
alert (regExpression.test("11aa"));

The above code is working fine.

But if we replace line 2 by line 3 then i开发者_JS百科t is not working

why? i am in a situation like I want to create the var only by appending(the expression come dynamically) so what should i do?

Line 3 sets regExpression to a string. Strings does not have a test method. You need to turn the string into a RegExp.

var regExpression = new RegExp("^([a-zA-Z0-9_\\-\\.]+)$");

Omit the slashes, as they are not part of the regexp itself.

If you like to create a RegExp dynamically, use new RegExp(). This allows you to build the expression with string-functions