JavaScript String replace - How do you use matched variables in the replacement string?

Question

How do you use the matched variables in the pattern in the replacement string?

var regexp = new RegExp('needle', 'ig');
str.replace(regexp, '<span class="marked">//1</span>')

I think you should have a read of this codinghorror.com/blog/2009/11/parsing-html-the-cthulhu-way.html — Peter Kelly
– Peter Kelly, Commented Aug 9, 2011 at 8:46
@Peter Kelly: I'm not sure this is valid - his example does not reference parsing HTML, but instead deals with adding HTML tags around a string. Which is a perfectly valid use for a Regex. — OverZealous
– OverZealous, Commented Aug 9, 2011 at 8:47
Where did you see that you could use //1? Read the documentation on the replace string more closely, especially the part about the special $ variables. — user663031
– user663031, Commented Oct 28, 2016 at 14:03

Eineki · Accepted Answer · 2016-10-28 13:47:03Z

28

try

var regexp = new RegExp(something, 'ig');
str.replace(regexp, '<span class="marked">$&</span>')

References:

edited Oct 28, 2016 at 13:47

answered Aug 9, 2011 at 8:47

Eineki

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3 Comments

clarkk Over a year ago

it works.. but why $& ? what if you got some groups in the pattern.. $1 etc doesnt seem to work

OverZealous Over a year ago

Interesting. I actually didn't know about the $&, $' and $` replacements.

Eineki Over a year ago

@claekk: $& represent the matched string, $n represent the nth group matched. If you don't use group matching should be preferable ($1 should work too, though). See the reference I added

OverZealous · Accepted Answer · 2011-08-09 08:52:35Z

7

The correct way to use backreferences in JavaScript is via $1...$9.

To make your example work:

var regexp = new RegExp(something, 'ig');
var result = str.replace(regexp, '<span class="marked">$1</span>');

More information is available here: http://www.regular-expressions.info/javascript.html#replace

edited Aug 9, 2011 at 8:52

answered Aug 9, 2011 at 8:44

OverZealous

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1 Comment

OverZealous Over a year ago

To use the numeric backreferences, you'll need to wrap your regex in parentheses: /(needle)/. Otherwise, use $& that Eineki shows.

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