How do I compress space characters with PHP regular expressions?

$string = 'Hello    this is a bunch of numbers: 333 and letters and $pecial Characters@!*(';

$foo = preg_replace("/[^a-zA-Z0-9\s]/", "", $string);

echo $foo;

The above returns:

Hello    this is a bunch of numbers 333 and letters and pecial Characters

I want to retain spaces but not if theres more than one. How can that be done?

It should look like:

开发者_如何学编程Hello this is a bunch of numbers 333 and letters and pecial Characters

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$foo = preg_replace("/[^a-zA-Z0-9\s]/", "", $string);
$foo = preg_replace('/\s+/',' ',$foo);

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One regex will do it:

$foo = preg_replace('/[^a-zA-Z0-9\s]|(\s)\s+/', '$1', $string);

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I haven't worked with PHP, but in Perl it's something like:

s/\s+/ /g

i.e. replace any sequence of one or more spaces with a single space.

So I imagine the PHP to compress spaces would be:

$foo = preg_replace("/\s{2,}/", " ", $string);

I don't think there should be any problems with running two preg_replace lines, especially if it makes the code clearer.

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Replace globally (edit - tested):

/(?<=\s)\s+|[^a-zA-Z0-9\s]+/

with ""