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<?php $tag = 'p'; var_dump(preg_match('/^[\\w-:]+$/', $tag));
Output for 7.3.0 - 7.3.5
Warning: preg_match(): Compilation failed: invalid range in character class at offset 4 in /in/vD5O2 on line 5 bool(false)
Output for 4.3.0 - 7.2.18
int(1)