Match dynamic string using regex

Question

I'm trying to detect an occurrence of a string within string. But the code below always returns "null". Obviously something went wrong, but since I'm a newbie, I can't spot it. I'm expecting that the code returns "true" instead of "null"

var searchStr = 'width'; 
var strRegExPattern = '/'+searchStr+'\b/'; 
"32:width: 900px;".match(new RegExp(strRegExPattern,'g'));

Answer 1

Please don't put '/' when you pass string in RegExp option

Following would be fine

var strRegExPattern = '\\b'+searchStr+'\\b'; 
"32:width: 900px;".match(new RegExp(strRegExPattern,'g'));

Answer 2

Notice that '\\b' is a single slash in a string followed by the letter 'b', '\b' is the escape code \b, which doesn't exist, and collapses to 'b'.

Also consider escaping metacharacters in the string if you intend them to only match their literal values.

var string = 'width';
var quotemeta_string = string.replace(/[^$\[\]+*?.(){}\\|]/g, '\\$1'); // escape meta chars
var pattern = quotemeta_string + '\\b';
var re = new RegExp(pattern);
var bool_match = re.test(input); // just test whether matches
var list_matches = input.match(re); // get captured results

Answer 3

The right tool for this job is not regex, but String.indexOf:

var str = '32:width: 900px;',
    search = 'width',
    isInString = !(str.indexOf(search) == -1); 

// isInString will be a boolean. true in this case

Documentation: https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Objects/String/indexOf

Answer 4

You're mixing up the two ways of creating regexes in JavaScript. If you use a regex literal, / is the regex delimiter, the g modifier immediately follows the closing delimiter, and \b is the escape sequence for a word boundary:

var regex = /width\b/g;

If you create it in the form of a string literal for the RegExp constructor, you leave off the regex delimiters, you pass modifiers in the form of a second string argument, and you have to double the backslashes in regex escape sequences:

var regex = new RegExp('width\\b', 'g');

The way you're doing it, the \b is being converted to a backspace character before it reaches the regex compiler; you have to escape the backslash to get it past JavaScript's string-literal escape-sequence processing. Or use a regex literal.