Suddenly, how does the php regular expression match the specified format?

Suddenly, how does the php regular expression match the specified format? I don't know the simple requirement, but I don't know which function to use.

I asked the user to enter only letters, but not any other characters: I used preg_match. after careful consideration, preg_match only matches whether the user input contains letters rather than letters,
For example, if the user inputs 123abcd456, if I use preg_match, the user inputs are correct, but this is obviously incorrect. what I want is that the user inputs can only contain letters instead of letters, which function should I use?

Thank you.

Reply to discussion (solution)

/^ [A-z] + $/I only match letters

/^ [A-z] + $/I only match letters

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