Javascript won't split using regex

Question

Since I started writing this question, I think I figured out the answers to every question I had, but I thought I'd post anyway, as it might be useful to others and more clarification might be helpful.

I was trying to use a regular expression with lookahead with the javascript function split. For some reason it was not splitting the string even though it finds a match when I call match. I originally thought the problem was from using lookahead in my regular expression. Here is a simplified example:

Doesn't work:

"aaaaBaaaa".split("(?=B).");

Works:

"aaaaBaaaa".match("(?=B).");

It appears the problem was that in the split example, the passed string wasn't being interpreted as a regular expression. Using forward slashes instead of quotes seems to fix the problem.

"aaaaBaaaa".split(/(?=B)./);

I confirmed my theory with the following silly looking example:

"aaaaaaaa(?=B).aaaaaaa".split("(?=B).");

Does anyone else think it's strange that the match function assumes you have a regular expression while the split function does not?

I've always used the /regex/ (no quotes) syntax for everything: match, split, replace. I didn't realize you didn't have to in some cases. — Mark Biek
– Mark Biek, Commented May 1, 2009 at 16:40
Actually it makes sense to let match() assume a regex (what else would there be that you can match against?) and split() assume a string, since splitting on a string is probably faster than splitting on a regex. Only if you pass an actual regex to split(), it will do the slower regex splitting. — Tomalak
– Tomalak, Commented May 1, 2009 at 17:01

James · Accepted Answer · 2009-05-01 16:58:56Z

31

String.split accepts either a string or regular expression as its first parameter. The String.match method only accepts a regular expression.

I'd imagine that String.match will try and work with whatever is passed; so if you pass a string it will interpret it as a regular expression. The String.split method doesn't have the luxury of doing this because it can accept regular expressions AND strings; in this case it would be foolish to second-guess.

Edit: (From: "JavaScript: The Definitive Guide")

String.match requires a regular expression to work with. The passed argument needs to be a RegExp object that specifies the pattern to be matched. If this argument is not a RegExp, it is first converted to one by passing it to the RegExp() constructor.

answered May 1, 2009 at 16:58

James

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EvanK · Accepted Answer · 2009-05-01 16:43:56Z

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If I recall correctly (and I could be very wrong here), the split method was implemented in javascript before the regex engine was in wide use, so it's presumably for backward compatibility.

answered May 1, 2009 at 16:43

EvanK

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Steen Over a year ago

I think this holds; String.split was introduced in 1.1 and Regular Expressions in 1.2. Reading the docs about split() (here: wp.netscape.com/eng/mozilla/3.0/handbook/javascript/newfunc.htm), and the regex enhancements to split() here: web.archive.org/web/20010408055419/developer.netscape.com/docs/… (no direct link, look under 'Regular Expressions'->'String methods'

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