Java how to replace 2 or more spaces with single space in string and delete leading and trailing spaces

Try this:

String after = before.trim().replaceAll(" +", " ");

See also

• String.trim()
  • Returns a copy of the string, with leading and trailing whitespace omitted.
• regular-expressions.info/Repetition

No trim() regex

It’s also possible to do this with just one replaceAll, but this is much less readable than the trim() solution. Nonetheless, it’s provided here just to show what regex can do:

    String[] tests = {
        "  x  ",          // [x]
        "  1   2   3  ",  // [1 2 3]
        "",               // []
        "   ",            // []
    };
    for (String test : tests) {
        System.out.format("[%s]%n",
            test.replaceAll("^ +| +$|( )+", "$1")
        );
    }

There are 3 alternates:

• ^_+ : any sequence of spaces at the beginning of the string
  • Match and replace with $1, which captures the empty string
• _+$ : any sequence of spaces at the end of the string
  • Match and replace with $1, which captures the empty string
• (_)+ : any sequence of spaces that matches none of the above, meaning it’s in the middle
  • Match and replace with $1, which captures a single space

See also

• regular-expressions.info/Anchors