Python Regex Engine - "look-behind需要固定宽度模式"错误

Question

我试图在CSV格式的字符串中处理未匹配的双引号.

确切地说,

"It "does "not "make "sense", Well, "Does "it"

应该更正为

"It" "does" "not" "make" "sense", Well, "Does" "it"

所以基本上我要做的就是

替换所有'''

1. 前面没有行开头或逗号(和)
2. 后面没有逗号或行尾

与'""'

为此,我使用下面的正则表达式

(?<!^|,)"(?!,|$)

问题是,虽然红宝石正则表达式引擎(http://www.rubular.com/)都能够解析正则表达式,蟒蛇正则表达式引擎(https://pythex.org/,http://www.pyregex.com/)抛出以下错误

Invalid regular expression: look-behind requires fixed-width pattern

并且使用python 2.7.3它会抛出

sre_constants.error: look-behind requires fixed-width pattern

谁能告诉我这里有什么烦恼？

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编辑:

在Tim的回应之后,我获得了多行字符串的以下输出

>>> str = """ "It "does "not "make "sense", Well, "Does "it"
... "It "does "not "make "sense", Well, "Does "it"
... "It "does "not "make "sense", Well, "Does "it"
... "It "does "not "make "sense", Well, "Does "it" """
>>> re.sub(r'\b\s*"(?!,|$)', '" "', str)
' "It" "does" "not" "make" "sense", Well, "Does" "it" "\n"It" "does" "not" "make" "sense", Well, "Does" "it" "\n"It" "does" "not" "make" "sense", Well, "Does" "it" "\n"It" "does" "not" "make" "sense", Well, "Does" "it" " '

在每一行的末尾,在'它'旁边添加了两个双引号.

所以我对正则表达式做了一个非常小的改动来处理一个换行符.

re.sub(r'\b\s*"(?!,|$)', '" "', str,flags=re.MULTILINE)

但这给出了输出

>>> re.sub(r'\b\s*"(?!,|$)', '" "', str,flags=re.MULTILINE)
' "It" "does" "not" "make" "sense", Well, "Does" "it"\n... "It" "does" "not" "make" "sense", Well, "Does" "it"\n... "It" "does" "not" "make" "sense", Well, "Does" "it"\n... "It" "does" "not" "make" "sense", Well, "Does" "it" " '

最后一个'它'只有两个双引号.

但我想知道为什么'$'行尾字符不会识别该行已经结束.

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最后的答案是

re.sub(r'\b\s*"(?!,|[ \t]*$)', '" "', str,flags=re.MULTILINE)

Answer 1

Python lookbehinds确实需要固定宽度,当你在一个不同长度的lookbehind模式中进行交替时,有几种方法可以处理这种情况:

• 重写模式,这样你就不必使用替换(例如Tim使用单词边界的上面的答案,或者你也可以使用(?<=[^,])"(?!,|$)当前模式的完全等价物,它需要在双引号之前使用逗号而不是逗号,或者是常见的用于匹配用空格括起来的单词的模式(?<=\s|^)\w+(?=\s|$),可以写成(?<!\S)\w+(?!\S)),或
• 拆分lookbehinds:
  • 积极的外观需要在一个组中交替(例如(?<=a|bc)应该重写为(?:(?<=a)|(?<=bc)))
  • 消极的(?<!^|,)"(?!,|$)外观可以被连接起来(例如应该看起来像(?<!^)(?<!,)"(?!,|$)).

Answer 2

Python lookbehind断言需要固定宽度,但你可以试试这个:

>>> s = '"It "does "not "make "sense", Well, "Does "it"'
>>> re.sub(r'\b\s*"(?!,|$)', '" "', s)
'"It" "does" "not" "make" "sense", Well, "Does" "it"'

说明:

\b      # Start the match at the end of a "word"
\s*     # Match optional whitespace
"       # Match a quote
(?!,|$) # unless it's followed by a comma or end of string