RegEx for Username validation

im a very beginner in RegEx.

I search a RegEx that validates if a string (username) is at least 3 chars, has no nu开发者_运维知识库mber or special char (ü, ö, ä) in firstspace, and no other special char else then üöä in total string.

Any help please?

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^[a-zA-Z][a-zA-Z0-9üöä]{2,}$