Description:
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The substitution variable of mb_ereg_replace() is not working when a substitution variable is preceded by a backslash

If you look at https://3v4l.org/OjhAe the output is different for prior to PHP 7.3.0.

Test script:
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<?php

var_dump(mb_ereg_replace('(%)', '\\\1', 'a%c'));

Expected result:
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string(4) "a\%c"

Actual result:
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string(5) "a\\1c"