Match exactly one of each from set of characters

Question

I am trying to match a specific set of characters but 1 and only 1 of each.

For example if the set is [abcd], I want to match string containing these exact characters in any order.

abcd  - true
bcad  - true
abc   - false (need all characters)
abbd  - false
abcdd - false

From what I understand so far there is no easy way to achieve this with RegEx but no answer was conclusive enough.

What language are you using? There's no *pattern* in your case, I doubt if regex is the best approach (of course you can use alternation on all combinations.. but that would be horrible). — Maroun, Mar 14 '16 at 14:17

score 9 · Accepted Answer · edited May 23 '17 at 10:30

9

I would think of capturing and using a lookahead to check if the same character is not ahead.

\b(?:([abcd])(?!\w*?\1)){4}\b

(?: opens a non capture group for repetition
\b matches a word boundary
([abcd]) captures one of [abcd]
(?!\w*?\1) checks if the captured character is not ahead with any amount of \w in between
{4}\b 4 times until another word boundary

See demo at regex101 (works only, if a lookahead is available in your regex flavor)

edited May 23 '17 at 10:30

Community

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answered Mar 14 '16 at 14:26

bobble bubble

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I think this is best regex solution for OP ++ – anubhava Mar 14 '16 at 14:43

score 2 · Answer 2 · answered Mar 14 '16 at 14:19

2

You can use a lookahead for each character to assert that char is present only once in your regex.

So e.g. for your input chars abcd generate this regex:

^(?=[^a]*a[^a]*$)(?=[^b]*b[^b]*$)(?=[^c]*c[^c]*$)(?=[^d]*d[^d]*$)

RegEx Demo

^(?=[^a]*a[^a]*$) will assert that a is present only once in input. and so on for other chars b,c,d.

answered Mar 14 '16 at 14:19

anubhava

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What if the input is `abcdefghijklmnopqrstuvwxyz` ? – Mar 14 '16 at 14:20
It will obviously fail since `x,y,z` etc are not present in input but if input has too many characters then generated regex might be too long as well – anubhava Mar 14 '16 at 14:21
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It doesn't look beautiful but should do the job efficiently. – bobble bubble Mar 14 '16 at 14:39

score 1 · Answer 3 · answered Mar 14 '16 at 14:23

The problem you're trying to solve is checking whether a string is a permutation of "abcd".

You don't want to use regex for every problem you have. It's useful and powerful when you have a pattern. Checking if a string is a permutations of "abcd" is not really a "pattern".

The best approach would be using the language's power and construct a solution from the functions available for you.

score 1 · Answer 4 · answered Mar 14 '16 at 14:31

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Just a humble contribution using grep:

grep -Px '[abcd]{4}' | grep -vP '(.).*\1'

get a seq of 4 [abcd]
and remove the lines contining repetitions

answered Mar 14 '16 at 14:31

JJoao

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score 1 · Answer 5 · answered Jun 06 '18 at 21:50

See bobble bubble's excellent answer for a real regex solution (which is sometimes necessary). This answer uses sorting and string equality instead of a single regex:

var set = 'bcad'; // in any order
var tests = [
  'abcd',  // true
  'bcad',  // true
  'abc',   // false (need all characters)
  'abbd',  // false
  'abcdd', // false
];

var setSorted = set.split('').sort().join();

for (var i = 0; i < tests.length; i++) {
  var pass = setSorted == tests[i].split('').sort().join();
  console.log(`${tests[i]}: ${pass}`);
}

Match exactly one of each from set of characters

5 Answers5