Solve specific PHP string expansion problem

I have a string composed of letters and square brackets ONLY. The square brackets define groups of letters. One letter from each group can be chosen in each expansion. I need to expand these letters into its actual strings. From this: $str = 'ABCCDF[GH]IJJ[KLM]' To this: $sub[0] = 'ABCCDFGIJJK'; $sub[1] = 'ABCCDFHIJJK'; $sub[2] = 'ABCCDFGIJJL'; $sub[3] = 'ABCCDFHIJJL'; $sub[4] = 'ABCCDFGIJJM'; $sub[5] = 'ABCCDFHIJJM'; The solution should be in PHP, compatible with 7.2 Should use no external libraries, only core PHP I expect a function with a string as input and an array of strings as a return function (string $string) : array { } Should work with strings like the example but no limit in the number of groups or in the number of letters in each group. Should be a generic function. Could be a recursive function. The letters between brackets are groups, and you can choose only one (1) of these letter for each expansion. You should do the corresponding permutations Other valid examples the function should solve are: $str = 'AB[ST]DF[GH]IJJ[KLM]A' To this: $sub[0] = 'ABSDFGIJJKA'; $sub[1] = 'ABSDFHIJJKA'; $sub[2] = 'ABSDFGIJJLA'; $sub[3] = 'ABSDFHIJJLA'; $sub[4] = 'ABSDFGIJJMA'; $sub[5] = 'ABSDFHIJJMA'; $sub[6] = 'ABTDFGIJJKA'; $sub[7] = 'ABTDFHIJJKA'; $sub[8] = 'ABTDFGIJJLA'; $sub[9] = 'ABTDFHIJJLA'; $sub[10] = 'ABTDFGIJJMA'; $sub[11] = 'ABTDFHIJJMA'; For example: A[BC]D should be expanded in: ABD ACD Another example AA[DCF] should be expanded in: AAD AAC AAF Another example with many groups AF[RT]SD[ER]D should be expanded in: AFRSDED AFTSDED AFRSDRD AFTSDRD One las example [AB] Should be expanded in: A B Strings without brackets are returned as is: AABCDFGTY is expanded in AABCDFGTY Examples of illegal (never expected strings) YOU SHOULD NOT HANDLE THIS CASES BECAUSE I TAKE CARE OF INPUT VALIDATION, THIS INFO IS ONLY FOR CLARITY: [A[B]]CCDD -> never nested brackets AB[CCD]FGG -> never groups with a repeated letter ADFG$% HJ -> characters different from letters and square brackets NOTE: * The returned array elements order is not important * There should not be exception handling in the input side. I take care of that. The input string is only letters [A-Z] and symbols '[' and ']' * Groups could be at the very beginning and/or at the very end then '[AE]DDFFGV[FGH]' is a valid string to be parsed. * Letters inside one group will never be repeated, for example, AB[CCD]E is not valid input and you don't need to check this case because I handle it.