How to put variable in regular expression match?

I have the following snippet. I want to find the appearance of a, but it does not work. How can I put the variable right?

var string1 = 'asdgghjajakhakhdsadsafdgawerwweadf';
var string2 = 'a';
string1.match('/' + string2 + '/g').length;

Solution 1:

You need to use the RegExp constructor instead of a regex literal.

var string = 'asdgghjjkhkh';
var string2 = 'a';
var regex = new RegExp( string2, 'g' );
string.match(regex);

If you didn't need the global modifier, then you could just pass string2, and .match() will create the regex for you.

string.match( string2 );

If you are merely looking to check whether a string contains another string, then your best bet is simply to use match() without a regex.

You may object: But I need a regex to check for classes, like \s, to define complicated patterns, etc..

In that case: You will need change the syntax even more, double-escaping your classes and dropping starting/ending / regex indicator symbols.

Imagine this regex...

someString.match(/\bcool|tubular\b);

The exact equivalent of this, when using a new new RegExp(), is...

someStringRegex = new RegExp('\\bcool|tubular\\b');

Two things happened in this transition:

Drop the opening and closing / (otherwise, your regex will fail).
Double escape your character classes, like \b becomes \\b for word borders, and \w becomes \\w for whitespace, etc. (otherwise, your regex will fail).