Python: Match a string that contains only upper and lowercase letters, numbers, and underscores - w3resource
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Username accepts upper case characters when creating an account · Issue #7921 · mattermost/mattermost-server · GitHub
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How to validate if input in input field has lowercase letters only using express-validator ? - GeeksforGeeks
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Universal Login Custom Template Username Validation Allowing Only Lowercase Alphanumeric Values - Auth0 Community
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Mistake in help text - should be underscore NOT hyphens · Issue #20 · backdrop-contrib/fanciblock · GitHub
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gwt - Good Code Practice - Why does eClipse force us to use "only lower-case letters, numbers and !" for Token name? - Stack Overflow
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