Regex for [a-zA-Z0-9\-] with dashes allowed in between but not at the start or end

Question

Update:

This question was an epic failure, but here\'s the working solution. It\'s based on Gumbo\'s answer (Gumbo\'s was close to working so I chose it as the accepte

Answer 1

Try this regular expression:

^[a-zA-Z0-9]+(-[a-zA-Z0-9]+)*$

This regular expression does only allow hyphens to separate sequences of one or more characters of [a-zA-Z0-9].

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Edit    Following up your comment: The expression (…)* allows the part inside the group to be repeated zero or more times. That means

a(bc)*

is the same as

a|abc|abcbc|abcbcbc|abcbcbcbc|…

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Edit    Now that you changed the requirements: As you probably don’t want to restrict each hyphen separated part of the words in its length, you will need a look-ahead assertion to take the length into account:

(?=[a-zA-Z0-9-]{4,25}$)^[a-zA-Z0-9]+(-[a-zA-Z0-9]+)*$