Match any horizontal whitespace characters at the start of each line in multiline strings in PHP

Question

I want to replace all empty spaces on the beginning of all new lines. I have two regex replacements:

$txt = preg_replace("/^　+/m", '', $txt);
$txt = preg_replace("/^[^\S\r\n]+/m", '', $txt);

Each of them matches different kinds of empty spaces. However, there may be chances that both of the empty spaces exist and in different orders, so I want to match occurences of all of them at the beginning of new lines. How can I do that?

NOTE: The first regex matches an ideographic space, \u3000 char, which is only possible to check in the question raw body (SO rendering is not doing the right job here). The second regex matches only ASCII whitespace chars other than LF and CR. Here is a demo proving the second regex does not match what the first regex matches.

@revo, thanks, but when I am using ^\h+, all the content in $txt get replaced. Any idea why? — user2335065, May 06 '19 at 09:35
Try `"/^\h+/mu"`. Your first regex matches an ideographic space, so you deal with Unicode texts, and `u` modifier might be handy here. — Wiktor Stribiżew, May 06 '19 at 09:36
@WiktorStribiżew @revo Oh I mistyped something. Adding the `u` does work, thanks! — user2335065, May 06 '19 at 09:47
FYI your second regex `^[^\S\r\n]+` includes what first regex matches. It means the second regex does all you want, requiring no `u` flag. — revo, May 06 '19 at 10:31

score 2 · Accepted Answer · answered May 06 '19 at 10:13

Since you want to remove any horizontal whitespace from a Unicode string you need to use

\h regex escape ("any horizontal whitespace character (since PHP 5.2.4)")
u modifier (see Pattern Modifiers)

Use

$txt = preg_replace("/^\h+/mu", '', $txt);

Details

^ - start of a line (m modifier makes ^ match all line start positions, not just string start position)
\h+ - one or more horizontal whitespaces
u modifier will make sure the Unicode text is treated as a sequence of Unicode code points, not just code units, and will make all regex escapes in the pattern Unicode aware.

Match any horizontal whitespace characters at the start of each line in multiline strings in PHP

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