Groovy: Enhancement to String Class to Add a Regexp Find Method

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I just submitted a groovy patch that enhances the String class with a “find” method that makes working with regular expressions much easier.

One of the most common use cases is to search a string for a regular expression pattern. If a match is found, then do something with the matched value.

Currently in groovy, the recommended way to do this is to create a matcher and then use indexes to work with any matches that might be found:

assert "10292" == ("New York, NY 10292" =~ /\d{5}/)[0]

If you try to do that on a string that doesn’t actually match the regular expression, you’ll get an IndexOutOfBoundException. To be safe, you need to check matcher.find() (not “matches” as that requires the entire string to match!) to see if the string is actually in there:

def m = ("New York, NY" =~ /\d{5}/)[0]
def zip
if (m.find()) {
   zip = m[0]

It also has inconsistent behavior if the regular expression happens to have capture groups in it. Then it returns an array containing the match and the capture groups, forcing you to index into that array to actually get the match you want:

assert "c" == ("foo car baz" =~ /(.)ar/)[0][1]

Groovy has already added closure aware replace method to the String class. The patch adds a complimentary find method to string that will return the string matched by the closure without needing to worry about matcher objects and array indexes.

You can either call it without a closure to get the full found match back (even if it has groups in it):

assert "10292" == "New York, NY 10292".find(/\d{5}/)

It safely returns a null if the match isn’t found, which can clean up boilerplate safety checks quite a bit. The user can check for null using groovy truth if they want to:

def zip = "New York, NY".find(/\d{5}/)   // returns null

if (zip) { ... }

If you want to work with capture groups, or manipulate the value, you can pass a closure to the find method that will be passed the full match as well as any capture groups (just as the collection based regular expression methods work):

// no capture groups, only the match is passed to the closure
assert "bar" == "foo bar baz".find(/.ar/) { match -> return match }

// one capture group
assert "b" == "foo bar baz".find(/(.)ar/) { match, firstLetter -> return firstLetter }

// many capture groups, all passed to the closure after the full match
assert "2339999" == "adsf 233-9999 adsf".find(/(\d{3})?-?(\d{3})-(\d{4})/) { match, areaCode, exchange, stationNumber ->
    assert "233-9999" == match
    assert null == areaCode
    assert "233" == exchange
    assert "9999" == stationNumber
    return "$exchange$stationNumber"

If you think this would be a valuable addition to groovy, you can vote for it in JIRA.