Python regex: string does not contain "jpg" and must have "-" and lowercase

I'm having troubles figuring out a python regex for django urls. I have a certain criteria, but can't seem to come up with the magic formula. In the end its so I can identify which page is a CMS page and pass to the django function the alias url it should load.

Here are some examples of valid strings which would match:

• about-us
• contact-us
• terms-and-conditions
• info/learn-more-pg2
• info/my-example-url

Criteria:

• Must be all lowercase
• Must开发者_运维技巧 contain a dash "-"
• Can contain numbers, letters and a slash "/"
• Must be at least 4 characters long and a max of 30 characters
• Cannot contain special characters
• Cannot contain the words:
  • .jpg
  • .gif
  • .png
  • .css
  • .js

Examples which should not match:

• About-Us (has upper case)
• contactus (doesn't have a dash)
• pg (less than 4 characters)
• img/bg.gif (contains ".gif")
• files/my-styles.css (contains ".css")
• my-page@ (has a character other than letters, numbers, dash or slash)

I know this isn't even close yet, but this is as far as I've gotten:

(?P<alias>([a-z/-]{4,30}))

I apologize for having large requirements, but I just can't get my head wrapped around this regex stuff.

Thanks!

I'm puzzled as to why several of the commentators find that this is hard to do in a regex. This is exactly what regular expressions are good at.

if re.match(
    r"""^             # match start of the string
    (?=.*-)           # assert that there is a dash
    (?!.*\.(?:jpg|gif|png|css|js))  # assert that these words can't be matched
    [a-z0-9/-]{4,30}  # match 4-30 of the allowed characters
    $                 # match the end of the string""", 
    subject, re.VERBOSE):
    # Successful match at the start of the string
else:
    # Match attempt failed

It is true however that since the . isn't among the allowed characters, the check for the forbidden file extensions is not really necessary.

Here’s my first post on SO. Pleeaaase, correct my english whenever it will be needed, I do ask you.

I think that any of the following REs fits right:

'(?=.{4,30}\Z)(?=.*-)[-a-z0-9/]+\Z'

'(?=.{4,30}\Z)[a-z0-9/]\*-[-a-z0-9/]\*\Z'

'(?=.{4,30}\Z)(?:[a-z0-9/]+|)-[-a-z0-9/]*\Z'