python regex: don't allow a specific character to repeat

Question

I have a regex

^[a-z][a-z0-9\-]{6,10}[a-z0-9]$

Which matches the following rules:

8-12 characters in length
first character is lowercase alpha
last characters lowercase alpha or digit
internal characters can contain a hyphen

it's re-used a lot in a module, always alongside some other rules and regexes

while writing out some unit tests, i noticed that it's always used in conjunction with another specific rule.

hyphens may not repeat

i can't wrap my head around integrating that rule into this one. i've tried a few dozen approaches with lookbehinds and lookaheads, but have had no luck on isolating to the specific character AND keeping the length requirement.

You could add another test to see that it doesn't match "--" like: and not "--" in somestring — Ricardo Branco
– Ricardo Branco, Commented May 5, 2017 at 17:45
Don't try and put too much logic inside a regex. Just define a function that applies the first regex and then check for double hyphens afterwards. — Alex Hall
– Alex Hall, Commented May 5, 2017 at 17:46
That is how things are currently working. Unfortunately, someone inevitably invokes this regex instead of the combined function, and bad data gets through. I've had to do a dozen cleanups. — Jonathan Vanasco
– Jonathan Vanasco, Commented May 5, 2017 at 18:02

user557597 · Accepted Answer · 2017-05-05 17:46:56Z

7

No repeating hyphen ^[a-z](?:[a-z0-9]|-(?!-)){6,10}[a-z0-9]$

Explained

 ^ [a-z] 
 (?:
      [a-z0-9]   # alnum
   |             # or
      - (?! - )  # hyphen if not followed by hyphen
 ){6,10}
 [a-z0-9] $

answered May 5, 2017 at 17:46

user557597

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2 Comments

Jonathan Vanasco Over a year ago

thanks a ton. i never thought of using the | like that! I kept trying to work with something like [a-z0-9]+[a-z0-9\-] and trying to apply limits around that.

user557597 Over a year ago

@JonathanVanasco - No problem. Sometimes all you have to do is read out loud what a construct is. For classes, it's like a or b or c. The or is really alternations. So, you could always split them out into alternations without losing meaning. Once split out, the can be operated on independently (ie. qualified).

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