Java String.replaceAll not replacing concurrent repeated sequences

Question

I'm attempting to use String.replaceAll to chop some whitespace out of a string. However, when multiple concurrent instances of the regex pattern appear in the string, only every 2nd one is replaced. Very simple example: ``` String theString = "foo x x x x x bar"; String trimmed = theString.replaceAll("x\s*x", "xx"); System.out.println(theString); System.out.println(trimmed); ``` What I want to see: ``` foo x x x x x bar foo xxxxx bar ``` What I see: ``` foo x x x x x bar foo xx xx x bar ``` It seems that replaceAll doesn't consider the replacement text as a candidate for being itself replaced, and instead skips merrily onwards. Is there an easy fix for this?

Accepted Answer

The problem is that you match the `x` after the space; as such, after your first match, you match:

```
foo x x x x x bar
       ^
       |---- HERE
```

You don't want to swallow it; you have to use a lookahead:

```
.replaceAll("x\s+(?=x)", "x");
```

You could even go with both a lookahead and a lookbehind:

```
.replaceAll("(?<=x)\s+(?=x)", "");
```

(note that the `*` quantifier has been replaced with `+`; it allows not to match where there are no space characters, in which case you don't want to replace anyway)

Java String.replaceAll not replacing concurrent repeated sequences

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