正则表达式能否检测 ASCII 图像中垂直线结构中的三个 X？

确定是否存在三个 X 的垂直线形式

使用现代正则表达式风格，如 PCRE/PHP、Perl、.NET 或类似的，可以确定如果 ASCII“图像”中存在由三个 X 组成的垂直线。

考虑以下正则表达式：

(?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

此表达式使用具有自引用捕获组的前瞻来计算X 之前每行的字符数。它尝试通过匹配前瞻中指定的条件来匹配垂直格式中的三个 X 字符。

在线演示： https://regex101 .com/r/Xb5FXl/2

计算三个 X 的垂直线形成的数量

虽然它不能在像 Perl 和 PCRE 这样的有限后向的正则表达式风格中直接解决，但可以通过操作匹配的字符串来间接确定计数。

以下解决方案通过匹配和计算匹配中出现三个 X 的部分中的字符数来修改 m.buettner 的“部分 PCRE 解决方案”。

^
(?:
    (?:                   # match .+? characters
        .
        (?=               # counting the same number on the following two lines
            .*+\n
            ( ?+ . )
            .*+\n
            ( ?+ . )
        )
    )+?
    (?<= X )              # till the above consumes an X
    (?=                   # that matches the following conditions
        .*+\n
        ?+
        (?<= X )
        .*+\n
        ?+
        (?<= X )
    )
    (?=                   # count the number of matches
        .*+\n
        ( ?+ . )        # the number of matches = length of 
    )
)*                        # repeat as long as there are matches on this line
.*\n?                     # remove the rest of the line

此表达式与前一个表达式匹配相同的格式，但它添加了第三个捕获组并计算该组中匹配字符的长度。通过仅用第三个捕获组的内容替换匹配项，生成的字符串长度表示三 X 形式的数量。

Perl 中的示例用法：

$in =~ s/regex//gmx;
$count = length $in;

在线演示： https://regex101.com/r/iqxY1a/1

测试用例：

Test #0:
--------------------
X
X
X

result: 1 (X)


Test #1:
--------------------
..X....
..X....
..X....

result: 1 (.)


Test #2:
--------------------
..X.X..
..X.X..
....X..

result: 1 (.)


Test #3:
--------------------
..X....
..X....
...X...

result: 0 ()


Test #4:
--------------------
..X....
...X...
..X....

result: 0 ()


Test #5:
--------------------
....X..
.X..X..
.X.....

result: 0 ()


Test #6:
--------------------
.X..X..
.X.X...
.X.X...

result: 1 (.)


Test #7:
--------------------
.X..X..
.X..X..
.X..X..

result: 2 (.X)


Test #8:
--------------------
XXX
XXX
XXX

result: 3 (XXX)


Test #9:
--------------------
X.X.X
XXXXX
XXXXX
.X.X.

result: 5 (XXXXX)


Test #10:
--------------------
1....X.......
2..X..X...X....
3X.X...X..X.....
4X....XXXXXX.....
5X..XXX...........
6.....X..........
7.........X....X
8..X......X....X....
9..X......X....X....X...
A....X.....
B.X..X..
C.....
XXX
XXX
XXX
.

result: 8 (3458.XXX)

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