All of a sudden, PHP is exactly how to match whether the specified format is satisfied

Suddenly, how does PHP match the specified format?

Very simple request I actually do not know, do not understand which function to use.

The information I ask the user to enter is only a letter and cannot have any other characters: I'm using Preg_match, think carefully, preg_match just matches whether the user input contains letters, not just letters,
For example, user input 123abcd456, if I use Preg_match will get the user input is the correct conclusion, but this is obviously wrong, I want the user input can only be a letter, instead of containing, which function should be used AH?

Thank you

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------Solution--------------------
/^[a-z]+$/i matches only letters

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