Regex - match a string without leading and trailing spaces

depending on your regex engine supporting look around

^(?=[A-Za-z0-9])([A-Za-z0-9\s]*)(?<=[A-Za-z0-9])$

Demo

You could use lookarounds to ensure that there are no spaces at both ends:

^(?! )[A-Za-z0-9 ]*(?<! )$

Live demo

But if the environment doesn't support lookarounds the following regex works in most engines:

^[A-Za-z0-9]+(?: +[A-Za-z0-9]+)*$

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