replace multiple spaces in a string with a single space

Question

I have a file with has several spaces among the words at some point. I need to clean the file and replace existing multi-spaced sequences with one space only. I have written the following statement which does not work at all, and it seems I'm making a big mistake.

 $s = preg_replace("/( *)/", " ", $x);

My file is very simple. Here is a part of it:

Hjhajhashsh dwddd dddd sss   ddd wdd ddcdsefe xsddd   scdc yyy5ty    ewewdwdewde           wwwe ddr3r dce eggrg               vgrg fbjb   nnn  bh jfvddffv mnmb   weer ffer3ef f4r4 34t4 rt4t4t 4t4t4t4t    ffrr  rrr  ww w w ee3e iioi   hj   hmm  mmmmm mmjm lk ;’’ kjmm  ,,,, jjj hhh  lmmmlm m mmmm lklmm jlmm m

Answer 1

What I usually do to clean up multiple spaces is:

while (strpos($x, '  ') !== false) {
   $x = str_replace('  ', ' ', $x);
}

Conditions/hypotheses:

1. strings with multiple spaces are rare
2. two spaces are by far more common than three or more
3. preg_replace is expensive in terms of CPU
4. copying characters to a new string should be avoided when possible

Of course, if condition #1 is not met, this approach does not make sense, but it usually is.

If #1 is met, but any of the others is not (this may depend on the data, the software (PHP version) or even the hardware), then the following may be faster:

if (strpos($x, '  ') !== false) {
   $x = preg_replace('/  +/', ' ', $x); // i.e.: '/␣␣+/'
}

Anyway, if multiple spaces appear only in, say, 2% of your strings, the important thing is the preventive check with strpos, and you probably don't care much about optimizing the remaining 2% of cases.