Lost in translation - C# regex to JavaScript regex

Question

I'm having some trouble translating my working C# regular expression into JavaScript's regular expression implementation.

Here's the regular expression:

([a-z]+)((\d+)([a-z]+))?,?

When used on "water2cups,flour4cups,salt2teaspoon" you should get:

[
    ["water", "2cups", "2", "cups"]
    ["flout", "4cups", "4", "cups"]
    ["salt", "2teaspoon", "2", "teaspoon"]
]

... And it does. In C#. But not in JavaScript.

I know there are some minor differences across implementations. What am I missing to get this expression working in JavaScript?

Update

I am using the regex like so:

"water2cups,flour4cups,salt2teaspoon".match(/([a-z]+)((\d+)([a-z]+))?,?/g);

Re your update: If you use a RegExp#exec loop rather than String#match, you get the results you're expecting (see my answer). I'm not enough of a RegExp guru to tell you why. :-) — T.J. Crowder
– T.J. Crowder, Commented May 5, 2010 at 12:11

T.J. Crowder · Accepted Answer · 2010-05-05 12:46:24Z

Creating the RegExp

You haven't shown how you're creating your Javascript regular expression, e.g., are you using a literal:

var rex = /([a-z]+)((\d+)([a-z]+))?,?/;

or a string

var rex = new RegExp("([a-z]+)((\\d+)([a-z]+))?,?");

If the latter, note that I've escaped the backslash.

Global Flag

By default, Javascript regular expressions are not global, that may be an issue for you. Add the g flag if you don't already have it:

var rex = /([a-z]+)((\d+)([a-z]+))?,?/g;

or

var rex = new RegExp("([a-z]+)((\\d+)([a-z]+))?,?", "g");

Using `RegExp#exec` rather than `String#match`

Your edit says you're using String#match to get an array of matches. I have to admit I hardly ever use String#match (I use RegExp#exec, as below.) When I use String#match with your regex, I get...very odd results that vary from browser to browser. Using a RegExp#exec loop doesn't do that, so that's what I'd do.

Working Example

This code does what you're looking for:

var rex, str, match, index;

rex = /([a-z]+)((\d+)([a-z]+))?,?/g;
str = "water2cups,flour4cups,salt2teaspoon";

rex.lastIndex = 0; // Workaround for bug/issue in some implementations (they cache literal regexes and don't reset the index for you)
while (match = rex.exec(str)) {
    log("Matched:");
    for (index = 0; index < match.length; ++index) {
        log("&nbsp;&nbsp;match[" + index + "]: |" + match[index] + "|");
    }
}

(The log function just appends text to a div.)

My output for that is:

Matched:
  match[0]: |water2cups,|
  match[1]: |water|
  match[2]: |2cups|
  match[3]: |2|
  match[4]: |cups|
Matched:
  match[0]: |flour4cups,|
  match[1]: |flour|
  match[2]: |4cups|
  match[3]: |4|
  match[4]: |cups|
Matched:
  match[0]: |salt2teaspoon|
  match[1]: |salt|
  match[2]: |2teaspoon|
  match[3]: |2|
  match[4]: |teaspoon|

(Recall that in Javascript, match[0] will be the entire match; then match[1] and so on are your capture groups.)

npinti · Accepted Answer · 2010-05-05 11:28:08Z

1

C# had the "@" operator which automatically escapes backslashes (). I do not think that Javascript supports it, so you basically need to "escape" the backslash by putting in another one, so this should do the trick

([a-z]+)((\d+)([a-z]+))?,?

answered May 5, 2010 at 11:28

npinti

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1 Comment

T.J. Crowder Over a year ago

You only have to escape backslashes if you're using a string to create the regex, not if you're using literal notation.

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Creating the RegExp

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Using `RegExp#exec` rather than `String#match`

Working Example

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2 Answers 2

Creating the RegExp

Global Flag

Using RegExp#exec rather than String#match

Working Example

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Using `RegExp#exec` rather than `String#match`