/^[[:word:]]*[[:alpha:]][[:word:]]*$/
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This accepts words with two underscores, it isn't clear from your description whether more than one underscore is allowed. Basically, your requirements reduce to: letters, numbers, underscores, with at least one letter. Which means you have zero or more letters, numbers, and underscores, then a letter, then again zero or more letters, numbers, and underscores.