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<?php var_dump(mb_ereg_replace('(%)', '\\\1', 'a%c'));
Output for 7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.34, 7.3.2 - 7.3.33, 7.4.0 - 7.4.30, 8.0.0 - 8.0.22, 8.1.0 - 8.1.9
string(4) "a\%c"
Output for 7.3.0 - 7.3.1
string(5) "a\\1c"

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49.64 ms | 409 KiB | 5 Q