Description:
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The "\k" escape must be followed by 'name', <name> or {name}. A PCRE regex that contains "\k" without the properly quoted name will compile without an error and will always fail as "\k" can never (as far as I know) match anything in the subject string. It is almost identical to "\g", except "\g" also allows "\gn", "\g{n}" or "\g{-n}" where "n" is any positive decimal number.

I have been unable to test PCRE directly (i.e. via the command line, not through PHP) as I can't compile it on my computer. I have looked through the source code and the test results, and from that it seems to be working correctly. Although I'm not 100% sure, I think this bug is specific to PHP.

Test script:
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<?php
var_dump(preg_match('/\k/', 'foo')); // Missing named reference
var_dump(preg_match('/\g/', 'foo')); // Missing named or numbered reference
?>

Expected result:
----------------
Warning: preg_match(): Compilation failed: **something about name missing in \k back-reference** at offset 1 in test.php on line 3
 bool(false)
Warning: preg_match(): Compilation failed: a numbered reference must not be zero at offset 1 in test.php on line 3
 bool(false)

Actual result:
--------------
int(0) 
Warning: preg_match(): Compilation failed: a numbered reference must not be zero at offset 1 in test.php on line 3
 bool(false)