Match Regex Pattern which does not only consist of certain characters-CodePudding

I'm trying to match Strings up to a certain size which do not only consist of certain characters.

So for the first part it's rather easy. To match this:
Yes: "abcde / asbas"
Yes: "abcde/ asbas"
Yes: "abcde/ ///"
Yes: " ///aasd"
Yes: "/// aasd"
Yes: "avcdesc"
No: "AVASD/ASDB"
I use this expression:

[ \/a-z0-9]{1,20}

But this would also match something like this: " ////"
As I want to avoid this I tried to add another expression and combine them using Lookahead:

(?=(?![ \/] $))(?=[ \/a-z0-9]{1,20}).*$

This works regarding avoiding the previous example " ////"
But somehow everything character after valid characters is ignored, so the following behavior occurs:
A String like this "asdad988AAA" is matched. As explained, this is not intended. Maybe someone has an idea on how to solve this.
Thanks.
Regex101: https://regex101.com/r/nmSMMu/1

CodePudding user response：

You can use

^(?=[ \/]*[a-z0-9])[a-z0-9\/ ]{1,20}$

See the regex demo. Details:

^ - start of string
(?=[ \/]*[a-z0-9]) - there must be a lowercase ASCII letter or digit after any zero or more spaces or / chars
[a-z0-9\/ ]{1,20} - one to twenty lowercase ASCII letters, digits, / or spaces
$ - end of string.