PHP, allow certain characters

Question

I'm very confused with other things but how would I allow a username with a-z and capitals with numbers and spaces

Answer 1

https://stackoverflow.com/questions/10776391/php-allow-only-certain-characters-in-string-without-using-regex#10776434

https://stackoverflow.com/questions/10625904/only-allowing-certain-characters-in-a-string

Answer 2

That doesn’t work, already checked that.