Suddenly merged. how does php regular expression match to meet 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 only letters. for example, the user entered 123abcd4 and suddenly failed. 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 for sharing:
------ Solution --------------------
/^ [A-z] + $/I only match letters