FWIW, above does it all with a regex; alternatively you can just check the string being input is 3 digits long - $[0-9]{3}$ - and then use normal relational operators to check the number represented is > 0 and <= 400 [but regex’s are more fun! 
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Reminds me of a famous quote:
“Some people, when confronted with a problem, think ‘I know, I’ll use regular expressions.’ Now they have two problems.”
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