date_i18n bug with format M-t

[liedekef]

If the format "M-t" is used, the preg_replace used in date_i18n:

$dateformatstring = preg_replace( "/([^\\\])M/", "\\1" . backslashit( $datemonth_abbrev ), $dateformatstring );

is not matching the first character (being "M" in this case), resulting in the month abbrev not being replaced (the other regex's are similar).
A small php script that reproduces this (the format string starts with "M" and the second time with " ", the regex is taken from date_i18n):

<?php
$dateformatstring="M-t";
$dateformatstring = preg_replace( "/([^\\\])M/", "\\1" . "test", $dateformatstring );
print "WRONG result: ".$dateformatstring."\n";
$dateformatstring=" M-t";
$dateformatstring = preg_replace( "/([^\\\])M/", "\\1" . "test", $dateformatstring );
print "Correct result: ".$dateformatstring."\n";
?>

[pbearne]

looking at the code this not a problem and the incoming string has ' ' prefix

[liedekef]

If plugins call date_i18n() directly, this empty space as prefix is not a guarantee.

[pbearne]

start of unit test

[pbearne]

The extra space, is added in the function before regx and then removed after

<?php
$dateformatstring = ' '.$dateformatstring;
.....
$dateformatstring = substr( $dateformatstring, 1, strlen( $dateformatstring ) -1 );

not saying this is weird and maybe wrong

I can't see why we could create regx that doesn't need a leading space

I am working on a more complete set of unit tests and will post patch for that later

but this all work including your error

[Rarst]

I don't see a problem with regex, M-t works just fine for me as is right now.

Though I have noticed that specific locale might not have abbreviated month names, e.g. I see M replaced with Ukrainian locale, but not Russian as of right now.

[Rarst]

I've opened a new issue for locale thing: #44472