Matching a space in regex

If you're looking for a space, that would be " " (one space).

If you're looking for one or more, it's " *" (that's two spaces and an asterisk) or " +" (one space and a plus).

If you're looking for common spacing, use "[ X]" or "[ X][ X]*" or "[ X]+" where X is the physical tab character (and each is preceded by a single space in all those examples).

These will work in every* regex engine I've ever seen (some of which don't even have the one-or-more "+" character, ugh).

If you know you'll be using one of the more modern regex engines, "s" and its variations are the way to go. In addition, I believe word boundaries match start and end of lines as well, important when you're looking for words that may appear without preceding or following spaces.

For PHP specifically, this page may help.

From your edit, it appears you want to remove all non valid characters The start of this is (note the space inside the regex):

$newtag = preg_replace ("/[^a-zA-Z0-9 ]/", "", $tag);
#                                    ^ space here

If you also want trickery to ensure there's only one space between each word and none at the start or end, that's a little more complicated (and probably another question) but the basic idea would be:

$newtag = preg_replace ("/ +/", " ", $tag); # convert all multispaces to space
$newtag = preg_replace ("/^ /", "", $tag);  # remove space from start
$newtag = preg_replace ("/ $/", "", $tag);  # and end

Problem

1. Your regex '~^[A-Z]~' will match only the first capital letter. Check out Meta Characters in the Pattern Syntax for more information.

2. Your replacement is a newline character 'n' and not a space.

Solution

Use this code:

$String = 'ThisWasCool';
$Words = preg_replace('/(?<! )[A-Z]/', ' $0', $String);

The (?<! ) is an assertion that will make sure we don't add a space before a capital letter that already has a space before it.

Use a negative a lookbehind to define a boundary:

(?<!\)(?:\\)*|

See live demo here

Taking care of backslashes in Java, above regex would be:

(?<!\\)(?:\\\\)*\|