<?php $strings = [ 'Subject', // expect empty string '1234 Subject', // expect 1234 'Subject 5678', // expect 5678 '1234 Subject 5678', // expect 1234 'Test 1234 Subject 5678 test', // expect 1234 ]; var_export( preg_replace('/^\D+|\d+\K.*/', '', $strings) );
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