I have an alphabet $\Sigma = \{a,b,\#\}$. I'm trying to write a regex that only accepts strings that have the same pattern before and after the $\#$ without using back references. For example, $aab\#aab$ is accepted, but not $aba\#aab$. How in the world do you handle this? I thought you could maybe do something like $(a|b)^*\#(a|b)*$, but this doesn't work at all.

  • 6
    $\begingroup$ Exercise: Use the pumping lemma to show that this language is not regular. Advanced exercise: Use the pumping lemma to show that this language is not context-free. $\endgroup$
    – Pseudonym
    Commented Sep 23, 2021 at 4:33

1 Answer 1


As pointed out by Pseudonym (in the comments) it's not a known pattern nor regular to be matched by a simple regex. If you knew the possibilities then sure you could use a regex (aab#aab|bba#bba).

Another option is to write some code to use the regex to find a looser pattern but then validate the pattern that you wish to have:

const input = "aab#bba aab#aab"

 * @param {string} val input of plain text
 * @returns Array<string> Strings that have a matched pattern
function findMirroredString(val = "") {
  const matches = val.match(/([a-z]+#[a-z]+)/g);
  // Return early if no matches were found by the regex.
  if (!matches) return [];
  // Filter to find the correct patter of "x#y" where x equals y
  return matches.filter(match => {
      // Split the parts by the # symbol and check if the two parts are equal,
      // If they are equal then the pattern is valid.
      const parts = match.split("#");
      return parts[0] === parts[1];


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