编写正则表达式以匹配1个或2个字母，但不匹配全部3个字母

Question

我试图编写一个正则表达式来匹配包含x、y或z的字符串，但其中只能有1-2个

例如： 有效字符串=xxxx, xxxyyyy, xyxyx, zyzzzyyy, xzzzxx

无效字符串=xyz, xxxyyyyz, zxzyy

我最初是这样写的

regex = re.compile("((x*y*)*)|((x*z*)*)|(y*z*)*)")

我在这里的逻辑是，它首先用xy、xz、yz测试字符串。但不幸的是，这并不奏效。它适用于我的第一个测试字符串xyxyx，但对于我的第二个字符串zyzyzyzy，它不匹配。我是否以错误的方式使用垂直“或”线

Answer 1

我不太清楚你是如何得到你所得到的，但是如果你想匹配一个序列（只有x和y）或者（只有x和z）或者（只有y和z），你可以使用这样的表达式：

^([xy]*|[xz]*|[yz]*)$

字符类（方括号）是指定“这些字符中的任何一个”的方便方法。因此[xy]*表示“仅由x和y字符组成的任意长度的序列”

^和$（开始和结束）指示模式应该匹配整个字符串

此外，如果要防止""（空字符串）被匹配，可以将所有*替换为+

Answer 2

您需要断言单词的开始/结束边界\b，然后在三个不同的字符类之间进行转换|：

\b([xy]+|[zy]+|[xz]+)\b

Demo

您还可以使用更简单、更快的regex\b[xyz]+\b，并与Python逻辑结合使用：

[w for w in re.findall(r'\b[xyz]+\b', txt) if len(set(w))<=2]

Python Demo

Answer 3

使用前瞻确保包含三个（或更多）不同字符的任何字符串失败：

^(?!.*(.).*(?!\1)(.).*(?!\1|\2).)[xyz]+$

见proof

Python:

regex = r"^(?!.*(.).*(?!\1)(.).*(?!\1|\2).)[xyz]+$"

解释

                         EXPLANATION
--------------------------------------------------------------------------------
  ^                        the beginning of the string
--------------------------------------------------------------------------------
  (?!                      look ahead to see if there is not:
--------------------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
--------------------------------------------------------------------------------
    (                        group and capture to \1:
--------------------------------------------------------------------------------
      .                        any character except \n
--------------------------------------------------------------------------------
    )                        end of \1
--------------------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
--------------------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
--------------------------------------------------------------------------------
      \1                       what was matched by capture \1
--------------------------------------------------------------------------------
    )                        end of look-ahead
--------------------------------------------------------------------------------
    (                        group and capture to \2:
--------------------------------------------------------------------------------
      .                        any character except \n
--------------------------------------------------------------------------------
    )                        end of \2
--------------------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
--------------------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
--------------------------------------------------------------------------------
      \1                       what was matched by capture \1
--------------------------------------------------------------------------------
     |                        OR
--------------------------------------------------------------------------------
      \2                       what was matched by capture \2
--------------------------------------------------------------------------------
    )                        end of look-ahead
--------------------------------------------------------------------------------
    .                        any character except \n
--------------------------------------------------------------------------------
  )                        end of look-ahead
--------------------------------------------------------------------------------
  [xyz]+                   any character of: 'x', 'y', 'z' (1 or more
                           times (matching the most amount possible))
--------------------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string