RegEx - Finding multiple dashes in entire string... and a little more

user_5601253 0
Viewed 1796

So I am HORRIBLE with RegEx... can you help me with the following?

  • only allows 52 total characters
  • only has "a-z", "A-Z", "0-9", & "-" (letters, numbers, and dashes)
  • does not start with "-" (dash)
  • does not end with "-" (dash)
  • does not have "--" (two consecutive dashes together)
  • does not have more than 2 "-" (dashes) in the entire string (this is what I'm having problems with)

So here is a helpful list (I guess):

Pass:
    abc-123
    abc-123-abc
Fail: 
    -abc-123             (fails due to starting with a dash)
    abc-123-             (fails due to ending with a dash)
    abc-123-abc-123      (fails due to 3 dashes)
    abc-12#-abc          (failed due to having a character that is not a-z, A-Z, 0-9, or a dash)

This is what I currently have, but feel free to change it however you would like:

(?!.*--)^[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,50}[a-zA-Z0-9])?$

I'm sure there's a better way to do this, but as mentioned above, I'm horrible with expressions. My expression works, it just doesn't find more than two dashes.

Thanks for your help.

regex
2 Answers

You can use the following PCRE-flavoured regex:

/^(?!.*\-\-)(?!.*\-.+\-.*\-)(?!-)[a-z0-9-]{0,52}(?<!-)$/gmi

Demo

The regex can be made self-documenting by writing it in free-spacing mode:

/
^                    # match beginning of line
(?!.*\-\-.*$)        # the line may not contain two consecutive hyphens
(?!.*\-.*\-.*\-.*$)  # the line may not contain more than two hyphens
(?!-)                # the first char cannot be a hyphen               
[a-z0-9-]{0,52}      # match 0-52 letters, digits and hyphens
(?<!-)               # the last char cannot be a hyphen
$                    # match end of the line
/xgmi                # free-spacing, global, multiline, case indifferent modes

(?!.*\-\-.*$), (?!.*\-.*\-.*\-.*$) and (?!-) are negative lookaheads; (?<!-) is a negative lookbehind.

This matches each line of a string (convenient for showing test cases at the demo). If the string contains a single line the regex can be simplified somewhat:

\A(?!.*\-\-)(?!.*\-.+\-.*\-)(?!-)[a-z0-9-]{0,52}(?<!-)\z

Not that \A and \z are beginning and end of string anchors, whereas ^ and $ are beginning and end of line anchors. Compare the negative lookaheads in this regex with those in the earlier one.

Should it matter, this matches empty strings.

user_2569700

You could use assert a maximum of 52 chars in a positive lookahead.

Then match 1 or more times [a-zA-Z0-9]+ and repeat 0, 1 or 2 times or more times the same pattern preceded with a -

^(?=[a-zA-Z0-9-]{1,52}$)[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+){0,2}$

Explanation

  • ^ Start of line
  • (?= Positive lookahead, assert what is on the right is
    • [a-zA-Z0-9-]{1,52}$ Match any of the listed 1-52 times and assert end of string
  • ) Close lookahead
  • [a-zA-Z0-9]+ Match 1+ times any of the listed to prevent matching an empty string
  • (?: Non capture group
    • -[a-zA-Z0-9]+ Match - and 1+ times any of the listed without the -
  • ){0,2} Close group and repeat 0-2 times
  • $ End of line

Regex demo

user_54249880
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