Replacing spaces using regex in PHP

Question

I'm pretty new to regular expressions. I have a requirement to replace spaces in a piece of multi-line text. The replacement rules are these:

• Replace all spaces at start-of-line with a non-breaking space ( ).
• Replace any instance of repeated spaces (more than one space together) with the same number of non-breaking-spaces.
• Single spaces which are not at start-of-line remain untouched.

I used the Regex Coach to build the matching pattern:

/( ){2,}|^( )/

Let's assume I have this input text:

asdasd asdasd  asdas1
 asda234 4545    54
  34545 345  34534
34 345

Using a PHP regular expression replace function (like preg_replace()), I want to get this output:

asdasd asdasd  asdas1
 asda234 4545    54
  34545 345  34534
34 345

I'm happy doing simple text substitutions using regular expressions, but I'm having trouble working out how to replace multiple-times inside the match in order to get the output I desire.

Answer 1

I'd guess that it would be easier to find each space and replace it. To do that, use "look-ahead" and "look-behind" groups.

Or, find a space (\x20) that is either lead by or followed by any single whitespace (\s); but, only replace the space.

$str = "asdasd asdasd  asdas1\n asda234 4545    54\n  34545 345  34534\n34 345\n";

print preg_replace("/(?<=\s)\x20|\x20(?=\s)/", "&#160;", $str);

(I opted for #160 since markdown parses nbsp.)

Results in:

asdasd asdasd  asdas1
 asda234 4545    54
  34545 345  34534
34 345

For more info, check out PCRE and perlre.

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reply to comments

@Sprogz: At first, I thought the same. But the example shows a "\n " => "\n " between the 1st and 2nd lines.

Answer 2

You can use PHP's /e modifier to execute some code in the replacement, like this:

$str = preg_replace('/( {2,}|^ )/em', 'str_repeat(" ", strlen("\1"))', $str);

I've changed the regular expression to capture the spaces. The /m modifer puts it into multi-line mode, so ^ matches the start of any line.