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 accepted answer):

Solution:

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

Original Question (albeit, after 3 edits)

I’m using Python and I’m not trying to extract the value, but rather test to make sure it fits the pattern.

allowed values:

spam123-spam-eggs-eggs1
spam123-eggs123
spam
1234
eggs123

Not allowed values:

eggs1-
-spam123
spam--spam

I just can’t have a dash at the starting or the end. There is a question on here that works in the opposite direction by getting the string value after the fact, but I simply need to test for the value so that I can disallow it. Also, it can be a maximum of 25 chars long, but a minimum of 4 chars long. Also, no 2 dashes can touch each other.

Here’s what I’ve come up with after some experimentation with lookbehind, etc:

# Nothing here

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