Take the 2-minute tour ×

In Java, when doing a replaceAll to look for a regex pattern like:

replaceAll("\\?i\\b(\\w+)\\b(\\s+\\1)+\\b", "$1"); 

(to remove duplicate consecutive case-insensitive words, e.g. Test test), I'm not sure where I put the ?i. I read that it is supposed to be at the beginning, but if I take it out then i catch duplicate consecutive words (e.g. test test), but not case-insensitive words (e.g. Test test). So I thought I could add the ?i in the beginning but that does not seem to get the job done. Any thoughts? Thanks!

share|improve this question

5 Answers 5

up vote 27 down vote accepted

RegexBuddy is telling me if you want to include it at the beginning, this is the correct syntax:

share|improve this answer

Yes, case insensitivity can be enabled and disabled at will in Java regex.

It looks like you want something like this:

        "Have a meRry MErrY Christmas ho Ho hO"
            .replaceAll("(?i)\\b(\\w+)(\\s+\\1)+\\b", "$1")
    // Have a meRry Christmas ho

Note that the embedded Pattern.CASE_INSENSITIVE flag is (?i) not \?i. Note also that one superfluous \b has been removed from the pattern.

The (?i) is placed at the beginning of the pattern to enable case-insensitivity. In this particular case, it is not overridden later in the pattern, so in effect the whole pattern is case-insensitive.

It is worth noting that in fact you can limit case-insensitivity to only parts of the whole pattern. Thus, the question of where to put it really depends on the specification (although for this particular problem it doesn't matter since \w is case-insensitive.

To demonstrate, here's a similar example of collapsing runs of letters like "AaAaaA" to just "A".

        "AaAaaA eeEeeE IiiIi OoooOo uuUuUuu"
            .replaceAll("(?i)\\b([A-Z])\\1+\\b", "$1")
    ); // A e I O u

Now suppose that we specify that the run should only be collapsed only if it starts with an uppercase letter. Then we must put the (?i) in the appropriate place:

        "AaAaaA eeEeeE IiiIi OoooOo uuUuUuu"
            .replaceAll("\\b([A-Z])(?i)\\1+\\b", "$1")
    ); // A eeEeeE I O uuUuUuu

More generally, you can enable and disable any flag within the pattern as you wish.

See also

Related questions

share|improve this answer

You can also match case insensitive regexs and make it more readable by using the Pattern.CASE_INSENSITIVE constant like:

Pattern mypattern = Pattern.compile(MYREGEX, Pattern.CASE_INSENSITIVE);
Matcher mymatcher= mypattern.matcher(mystring);
share|improve this answer

If your whole expression is case insensitive, you can just specify the CASE_INSENSITIVE flag.


share|improve this answer

You also can lead your initial string, which you are going to check for pattern matching, to lower case. And use in your pattern lower case symbols respectively.

share|improve this answer

Your Answer


