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

Question

Looking for quick, simple way in Java to change this string

" hello     there   "

to something that looks like this

"hello there"

where I replace all those multiple spaces with a single space, except I also want the one or more spaces at the beginning of string to be gone.

Something like this gets me partly there

String mytext = " hello     there   ";
mytext = mytext.replaceAll("( )+", " ");

but not quite.

Answer 1

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