These are my PHP solutions for 2018's Advent of Code.
Tried to use eval() but felt dirty for doing so. Opted for preg_match() to
get the operator and add or subtract as necessary.
For part two, I just accumulated results in a "frequencies" array. Once I found a match, I simply returned the result.
I spent way too much time trying to find a regular expression that would match two and only
repeating characters. I resorted to a combination of count_chars() and array_unique() for part one.
In part two, I broke out my old pals similar_text() and array_intersect().