Ruby Regexp Not Working

Question

Following usernames should be allowed:

Foo-Bar
Foobar
Fooo_123
Foob123_1
Foo-bar-123

Following user names should not be allowed:

_Foobar_
F-o-o-b-a-r
-Foobar-
_-Foobar-_

This means: The string is allowed to be 3 to 20 characters long. There is only one dash or underscore allowed every three characters. Not at the beginning and not and the end. You are only allowed to use at most 2 of a dash or underscore, a maximum of 3 numbers, but a minimum of 3 letters.

This is the Regexp I've done so far, but I already failed at allowing dashes at the front and end:

/^[^\-_][a-zA-Z0-9]{3,20}[^\-_]$/

Thanks in advance!

tckmn · Accepted Answer · 2014-02-13 00:19:37Z

3

This is probably too complicated for a single regex, and if you could make one, it would be overly unreadable and complex. I suggest you simply have multiple checks; for example:

valid = str.length >= 3 && str.length <= 20        # or str.length.between? 3, 20
        && str =~ /^[^-_]+([-_][^-_]{3,})*[-_]?[^-_]+$/
        && str.count '-_' <= 2
        && str.count '0-9' <= 3
        && str.count 'A-Za-z' >= 3

Explanation of the regex:

/
  [^-_]+      # any amount of non-dashes/underscores (so it can't start with one)
  (
    [-_]      # a dash/underscore
    [^-_]{3,} # 3 or more non-dashes/underscores
  )
  *           # zero or more times
  [-_]?       # an optional dash/underscore
  [^-_]+      # any amount of non-dashes/underscores (so it can't end with one)
/x

edited Feb 13, 2014 at 0:19

answered Feb 12, 2014 at 23:10

tckmn

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7 Comments

the Tin Man Over a year ago

+1. Trying to make a kitchen-sink pattern is a slow descent into madness. I much rather prefer seeing several small, easily digestible, patterns, especially when I'm going to have to look at someone else's code.

Casimir et Hippolyte Over a year ago

Since you use the negative character class [^-_] the pattern allows any special characters (#!^$*%...). To ensure that there is only dashes, underscores, letters and digits you must change it to [a-zA-Z0-9] or [^\W_]. Another thing, why not using valid = str.length.between?(3, 20) (not tested)?

tckmn Over a year ago

@CasimiretHippolyte Where does it say special characters are not allowed? I suppose I could use between?; I'll edit the post

Casimir et Hippolyte Over a year ago

@Doorknob: You are right, it is said nowhere! I have assumed by the numbers that there isn't special characters by reading the example strings! However it can be a false assumption.

tckmn Over a year ago

@Just If you removed it, though, you wouldn't be able to use dashes or underscores at all!

|

Community · Accepted Answer · 2017-05-23 10:25:03Z

0

I would suggest you start with /^(?![-_])[-_a-zA-Z0-9]{3,20}(?<![-_])$/, which will allow all of your ok-usernames and only one of the not-ok (see rubular for details). Then continue as Doorknob suggested and impose limits on the frequencies - and _ are used, ...

How - and _ at beginning and end are suppressed?

(?![-_]) ensures the next character is neither a - nor _
(?<![-_]) ensures the previous character is neither a - nor _

edited May 23, 2017 at 10:25

CommunityBot

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answered Feb 12, 2014 at 23:50

TheConstructor

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