PHP Regex Check if two strings share two common characters

This is a regex that defines "valid" lines:

/^[A|B]{1}|[C|D]{1}|[E|F]{1}|[G|H]{1}$/m

In english, each line has either A or B at position 0, either C or D at position 1, either E or F at position 2, and either G or H at position 3. I can assume that each line will be exactly 4 characters long.

That's not what that regex means. That regex means that each line has either A or B or a pipe at position 0, C or D or a pipe at position 1, etc; [A|B] means "either 'A' or '|' or 'B'". The '|' only means 'or' outside of character classes.

Also, {1} is a no-op; lacking any quantifier, everything has to appear exactly once. So a correct regex for the above English is this:

/^[AB][CD][EF][GH]$/

or, alternatively:

/^(A|B)(C|D)(E|F)(G|H)$/

That second one has the side effect of capturing the letter in each position, so that the first captured group will tell you whether the first character was A or B, and so on. If you don't want the capturing, you can use non-capture grouping:

/^(?:A|B)(?:C|D)(?:E|F)(?:G|H)$/

But the character-class version is by far the usual way of writing this.

As to your problem, it is ill-suited to regular expressions; by the time you deconstruct the string, stick it back together in the appropriate regex syntax, compile the regex, and do the test, you would probably have been much better off just doing a character-by-character comparison.

I would rewrite your "ACFG" regex thus: /^(?:AC|A.F|A..G|.CF|.C.G|..FG)$/, but that's just appearance; I can't think of a better solution using regex. (Although as Mike Ryan indicated, it would be better still as /^(?:A(?:C|.E|..G))|(?:.C(?:E|.G))|(?:..EG)$/ - but that's still the same solution, just in a more efficiently-processed form.)

There are 6 possibilities that at least two characters match out of 4: MM.., M.M., M..M, .MM., .M.M, and ..MM ("M" meaning a match and "." meaning a non-match).

So, you need only to convert your input into a regex that matches any of those possibilities. For an input of ACFG, you would use this:

"/^(AC..|A.F.|A..G|.CF.|.C.G|..FG)$/m"

This, of course, is the conclusion you're already at--so good so far.

The key issue is that Regex isn't a language for comparing two strings, it's a language for comparing a string to a pattern. Thus, either your comparison string must be part of the pattern (which you've already found), or it must be part of the input. The latter method would allow you to use a general-purpose match, but does require you to mangle your input.

function findMatchingLines($line, $subject) {
  $regex = "/(?<=^([AB])([CD])([EF])([GH])[.\n]+)"
      + "(\1\2..|\1.\3.|\1..\4|.\2\3.|.\2.\4|..\3\4)/m";
  $matchingLines = array();
  preg_match_all($regex, $line + "\n" + $subject, $matchingLines);
  return $matchingLines;
}

What this function does is pre-pend your input string with the line you want to match against, then uses a pattern that compares each line after the first line (that's the + after [.\n] working) back to the first line's 4 characters.

If you also want to validate those matching lines against the "rules", just replace the . in each pattern to the appropriate character class (\1\2[EF][GH], etc.).