[java] Is Java RegEx case-insensitive?

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!

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

It looks like you want something like this:

    System.out.println(
        "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".

    System.out.println(
        "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:

    System.out.println(
        "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

• java.util.regex.Pattern
• regular-expressions.info/Modifiers
  • Specifying Modes Inside The Regular Expression
    • Instead of /regex/i (Pattern.CASE_INSENSITIVE in Java), you can do /(?i)regex/
  • Turning Modes On and Off for Only Part of The Regular Expression
    • You can also do /first(?i)second(?-i)third/
  • Modifier Spans
    • You can also do /first(?i:second)third/
• regular-expressions.info/Word Boundaries (there's always a \b between a \w and a \s)

Related questions

• Can you make just part of a regex case-insensitive?
• Case sensitive and insensitive in the same pattern

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

Pattern.compile(regexp, Pattern.CASE_INSENSITIVE)

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.

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);