Java: Regex to match when a string is present twice before and after a known string-CodePudding

I am looking for a java regular expression to match random string like this:

Apple for Apple
day for day
Money for Money
... etc.

So the regular expression should match the string if there is a "for" and an identical word before and after.

Currently i'm using this regex:

[A-Za-z] [ ]{1}(for){1}[ ]{1}[A-Za-z]

But it also returns wrong results like:

Apple for all
day for night
... etc.

I'd like to accomplish this with Regex only and no extra Java code. Is this possible?

CodePudding user response：

You can use groups to match the words before and after 'for'.

Try this: ^(\\w ) for \\1 Yes those are spaces between for, you can use '\s' too but then it will match all white space chars like \n, \r, \t etc.

Explanation:

^(\\w ) for \\1

^      beginning of the string
(\\w )  capture group to catch any word
       matches white space
for    matches 'for'
       matches white space
\\1     matches the word captured in the first capture group

Test the regex here: https://www.regexplanet.com/share/index.html?share=yyyyp3haukr

and here: https://regex101.com/r/wJvwob/1

CodePudding user response：

You may use String#matches() here with a backreference:

List<String> inputs = Arrays.asList(new String[] {"Apple for Apple", "Apple for Orange"});
for (String input : inputs) {
    if (input.matches("(\\w ) for \\1")) {
        System.out.println("MATCH   : "   input);
    }
    else {
        System.out.println("NO MATCH: "   input);
    }
}

This prints:

MATCH   : Apple for Apple
NO MATCH: Apple for Orange