如何在php中编写精确的模式匹配[重复]

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This question already has an answer here:

• how to use preg_match to check whole text 2 answers
• Why preg_match() always validates as true when partial match? 1 answer

I'm looking for the syntax for pattern matching or a function in PHP that returns true only if the pattern in the string occurs once. I only found the function preg_match. But in case preg_match('/[A-Za-z]{3}/', "asdf") it returns true, because the given string contains the pattern. I need the logic like it works in the html input tag in the attribute pattern. More or less symbols should return false.

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