How to use a regular expression on a variable to find a target?

Forum|Forum|2 days ago
June 4, 2026
1 reply
31 views

For example, I want to find numbers in the `source` variable.
The output is null?
What's wrong?

var source="@m2Title";

//Default regular expression
var re="(?<=@m)(\\d+)(?!\\d)"

//Standard Regular Expressions
var toReg=new RegExp(re);

var result=source.match(toReg);
alert(result);

Correct answer Peter Kahrel

Your second line defines just a string, not a regular expression. The third line isn’t necessary. The lookahead isn’t needed either in this case: \d+ continues until it runs into something that’s not a digit.

Also, JavaScript’s regular-expression engine doesn’t have lookbehind, so you’d do something like this:

var source="@m2Title";
//Default regular expression
var re=/@m(\d+)/;
var result=source.match(re);
alert(result);

Remember that ‘result’ is an array here.

Peter Kahrel

Correct answer

Community Expert

Your second line defines just a string, not a regular expression. The third line isn’t necessary. The lookahead isn’t needed either in this case: \d+ continues until it runs into something that’s not a digit.

Also, JavaScript’s regular-expression engine doesn’t have lookbehind, so you’d do something like this:

var source="@m2Title";
//Default regular expression
var re=/@m(\d+)/;
var result=source.match(re);
alert(result);

Remember that ‘result’ is an array here.

dubloveAuthor

Legend

Hi @Peter Kahrel

I only want numbers \d+

Thank you very much.

I found it:

var result=source.match(re)[1];

Peter Kahrel

Community Expert

This is very basic JavaScript regular-expression stuff. As it is, the regular expression returns null if there's no match, or a two-element array. The first element is the whole match (@m2), the second element is what was captured in the parentheses (2).

Change the alert as follows:

if (result == null) {
	alert('No result');
} else {
	alert (result[1]);
}

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