php regex surround string with parentheses unless already surrounded by non-literal parentheses

Question

I have some code that is augmenting a regular expression by combining horizontal white-space and surrounding it in parentheses, unless it is already surrounded by non-backslashed parentheses. I have a piece of code that's working:

$regex_find = preg_replace('/(?<!\()[ \t]{2,}(?!\))/', '([ \t]{2,})', $regex_find);

However, for starters, the look-behind segnment, (?<!\(), should really allow for a literal left parenthesis, \(, but not a stand-alone left parenthesis, (.

Thanks

Adjusting from Bart's comment, we have: $s = '(\( ) b c d'; $s = preg_replace('/([^\t (])([ \t]{2,})([^\t )])/', '$1([ \t]{2,})$3', $s); Firstly, the replace does not allow for \( preceding the core string. And, yes, this left parenthesis would have to be single backslashed only. In addition, the right parenthesis needs to be tied into the left one. A proper solution of the above test string would be: (\(([ \t]{2,})) b c([ \t]{2,})d. — beej
– beej, Commented Jul 13, 2011 at 22:19
Actually, any odd number of backslashes preceding the inner left parenthesis would create a literal parenthesis. — beej
– beej, Commented Jul 13, 2011 at 23:16

Jacob Eggers · Accepted Answer · 2011-07-14 16:53:47Z

1

Your original regex also has another problem. Try running it on this:

 $str = "asdf(     )sdf     sdf";
 //results in: "asdf( (   ) )sdf(     )sdf"

This regex should work:

$regex_find = '(    asdf(     )sdf     sdf';
preg_replace('/(?<!(\(|\s)(?=\s*\)))\s{2,}/','($0)',$regex_find);
//result = "((    )asdf(     )sdf(     )sdf"

Edit: Here is Qtax's solution with the additional lookbehind to ignore escaped \(

/(?<!((?<!\\)\(|\s)(?=\s*\)))\s{2,}/

edited Jul 14, 2011 at 16:53

answered Jul 13, 2011 at 22:09

Jacob Eggers

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2 Comments

Qtax Over a year ago

Great! But it's not checking escapes properly, could use another lookbehind, like /(?<!((?<!\\)$|\s)(?=\s*$))\s{2,}/: ideone.com/4cClT vs ideone.com/LmvVP

beej Over a year ago

Get "Warning: preg_replace() [function.preg-replace]: Compilation failed: missing ) at offset 33..." in php 4.4.9. Perhaps it's more trouble than it's worth. Nested parentheses aren't the worst thing

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