This is being conveyed is what meaning, which ace knows! ! !

/ ^(\w )(_1) $/ig
Which master knows, the trouble tells little younger brother\w matchs those who include underline any word character. Equal in value at [A-Za-z0-9_ ]
match in front child expression or for many times

Be like,can match: A_a_2A_a_2_1_1_1Up