Can Regular Expressions Be Used to Count Vertical Line Formations in ASCII Images?

Matching Vertical Patterns in an ASCII Image Using Regular Expressions

In the context of analyzing ASCII images or maps, situations may arise where identifying specific patterns becomes necessary. One such pattern is a vertical line formation of three Xs. Using regular expressions, it is possible to tackle this task effectively.

Question 1: Determining the Existence of a Vertical Line Formation

To determine if a vertical line formation of three Xs exists, the following regular expression can be employed:

(?xm)                    # ignore comments and whitespace, ^ matches beginning of line
^                        # beginning of line
(?:
    .                    # any character except \n
    (?=                  # lookahead
        .*+\n            # go to next line
        ( ?+ . )       # add a character to the 1st capturing group
        .*+\n            # next line
        ( ?+ . )       # add a character to the 2nd capturing group
    )
)*?                      # repeat as few times as needed
X .*+\n                  # X on the first line and advance to next line
?+                     # if 1st capturing group is defined, use it, consuming exactly the same number of characters as on the first line
X .*+\n                  # X on the 2nd line and advance to next line
?+                     # if 2st capturing group is defined, use it, consuming exactly the same number of characters as on the first line
X                        # X on the 3rd line

This expression utilizes lookaheads with self-referencing capturing groups to count the number of characters on each line before an X appears. If the pattern is detected, the expression will match successfully.

Question 2: Counting the Occurrences of Vertical Line Formations

While direct matching using regular expressions cannot provide an accurate count of vertical line formations due to limited lookbehind capabilities, there exists an indirect solution.

By replacing all matches of the following expression with $3, you can obtain the answer to question two (the number of patterns of interests) as the length of the resulting string:

^
(?:
    (?:                   # match .+? characters
        .
        (?=               # counting the same number on the following two lines
            .*+\n
            ( ?+ . )
            .*+\n
            ( ?+ . )
        )
    )+?
    (?<p>This expression employs a similar technique to the first question, but with modifications to include X in the characters matched in the first lookahead and wraps it with a quantifier to count the number of matches.</p><p>By utilizing the length of the resulting string as the count, this solution provides an indirect approach to answering question two.</p>

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