Matching Spaces with Regular Expressions in PHP

Finding space characters in a PHP regular expression can be done in several ways.

Single Space:
To match a single space, use " ":

preg_replace('/[ ]/', '', $tag);

Multiple Spaces:
To match one or more spaces, use " *":

preg_replace('/[ ] /', '', $tag);

Whitespace:
To match any whitespace, including tabs, use "\s":

preg_replace('/[\s]/', '', $tag);

Word Boundaries:
To match the start or end of a word where spaces are common, use "\b":

preg_replace('/\b[ ]\b/', '', $tag);

Remove Non-Valid Characters:
To remove all non-valid characters, leaving only letters, numbers, and spaces, use:

preg_replace('/[^a-zA-Z0-9 ]/', '', $tag);

Multiple Spaces between Words:
To remove multiple spaces between words while preserving single spaces, perform a series of replacements:

$newtag = preg_replace('/  /', ' ', $tag);
$newtag = preg_replace('/^ /', '', $newtag);
$newtag = preg_replace('/ $/', '', $newtag);

Note: The removal of spaces from the start and end is only necessary if the string may contain leading or trailing spaces.