Regex catching adjacent characters with a single character set

Question

I am trying to construct a regex statement that matches a string conforming to the following conditions:

3-63 lowercase alphanumeric characters, plus "." and "-"
May not start or end with . or -
Dashes and periods cannot be adjacent to each other.

abc-123.xyz <- should match

abc123-.xyz <- should not match

I have been able to put this regex together, but it does not catch the third requirement. I've tried to use another negative lookahead/lookbehind,[i.e. - (?!.-|-.) ] but its still matching the strings with adjacent periods and dashes. Here's the regex statement I came up with that fulfills conditions 1 & 2:

^(?!\.|-)([a-z0-9]|\.|-){3,63}(?<!\.|-)$

FYI, this regex is for validating input when specifiying an AWS S3 bucket name in a CloudFormation template.

Are `.` and `-` counted into the `3,63` character limit? Try `^(?=[a-z0-9.-]{3,63}$)[a-z0-9]+(?:[-.][a-z0-9]+)*$` — Wiktor Stribiżew, Aug 03 '17 at 14:26
How about this? `^(?:[a-zA-Z0-9]|-(?![.])|\.(?![-])){3,63}$` — Gurmanjot Singh, Aug 03 '17 at 15:04

Toto · Answer 1 · 2017-08-03T14:34:14.173

0

How about:

^(?=.{3,63}$)[a-z0-9]+(?:[-.][a-z0-9]+)*$

edited Aug 03 '17 at 14:34

answered Aug 03 '17 at 14:28

Toto

89,455
62
89
125

1

[`^(?=.{3,63}$)[a-z0-9]+(?:[-.]?[a-z0-9]+)*$` is a bad pattern](https://regex101.com/r/QNk8IB/1). See [here why](https://stackoverflow.com/questions/45463148/fixing-catastrophic-backtracking-in-regular-expression/45477009#45477009). – Wiktor Stribiżew Aug 03 '17 at 14:32
1

@WiktorStribiżew: Thanks for info. – Toto Aug 03 '17 at 14:34

score 0 · Answer 2 · answered Aug 03 '17 at 14:51

Use this Pattern ^(?!.*[.-](?=[.-]))[^.-][a-z0-9.-]{1,61}[^.-]$ Demo

#    ^(?!.*[.-](?=[.-]))[^.-][a-z0-9.-]{1,61}[^.-]$
^               # Start of string/line
(?!             # Negative Look-Ahead
  .             # Any character except line break
  *             # (zero or more)(greedy)
  [.-]          # Character in [.-] Character Class
  (?=           # Look-Ahead
    [.-]        # Character in [.-] Character Class
  )             # End of Look-Ahead
)               # End of Negative Look-Ahead
[^.-]           # Character not in [.-] Character Class
[a-z0-9.-]      # Character in [a-z0-9.-] Character Class
{1,61}          # (repeated {1,61} times)
[^.-]           # Character not in [.-] Character Class
$                # End of string/line

score -1 · Answer 3 · answered Aug 03 '17 at 14:31

^[a-z0-9](?:[a-z0-9]|[.\-](?=[a-z0-9])){2,62}$

We match a lowercase alphanumeric character, followed by between 2 and 62 repetitions of either:

a lowercase alphanumeric character, or
a . or - (which must be followed by a lowercase alphanumeric character).

The last restriction makes sure that you can't have two ./- characters in a row, or a ./- at the end of the string.

Regex catching adjacent characters with a single character set

3 Answers3