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<?php $tests = [ '7hello', 'Fooey', 'Shoo3y', 'Whoa!', 'A1_B2_C3_', '_ABC', 'UB40', 'omg', 'Hi_', 'Stackoverflow_', ]; foreach ($tests as $test) { printf( "%s : %s\n", $test, preg_match('/^(?:[a-z]\w*){3}$/i', $test) ? 'pass' : 'fail' ); }
Output for 8.0.1 - 8.0.30, 8.1.0 - 8.1.34, 8.2.0 - 8.2.30, 8.3.0 - 8.3.30, 8.4.1 - 8.4.18, 8.5.0 - 8.5.3
7hello : fail Fooey : pass Shoo3y : pass Whoa! : fail A1_B2_C3_ : pass _ABC : fail UB40 : fail omg : pass Hi_ : fail Stackoverflow_ : pass

preferences:
78.17 ms | 1121 KiB | 4 Q