高级正则表达式技术（Python版）

pasckr · 发表于 2016-5-9 14:35:34

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之前推荐过一篇：《最全的常用正则表达式大全》。虽然是跨年夜，但还是来给没约会的童鞋推荐推荐技术文章啦。

正则表达式是从信息中搜索特定的模式的一把瑞士军刀。它们是一个巨大的工具库，其中的一些功能经常被忽视或未被充分利用。今天我将向你们展示一些正则表达式的高级用法。

举个例子，这是一个我们可能用来检测电话美国电话号码的正则表达式：

r'^(1[-\s.])?([color=#333333 !important]$)?\d{3}(?(2)$)[-\s.]?\d{3}[-\s.]?\d{4}$'

我们可以加上一些注释和空格使得它更具有可读性。

r[color=#DD1144 !important]'^'
r[color=#DD1144 !important]'(1[-\s.])?'[color=#006FE0 !important] [color=#999999 !important]# optional '1-', '1.' or '1'
r[color=#DD1144 !important]'([color=#333333 !important]$)?'[color=#006FE0 !important]    [color=#999999 !important]# optional opening parenthesis
r[color=#DD1144 !important]'\d{3}'[color=#006FE0 !important]    [color=#999999 !important]# the area code
r[color=#DD1144 !important]'(?(2)$)'[color=#006FE0 !important] [color=#999999 !important]# if there was opening parenthesis, close it
r[color=#DD1144 !important]'[-\s.]?'[color=#006FE0 !important] [color=#999999 !important]# followed by '-' or '.' or space
r[color=#DD1144 !important]'\d{3}'[color=#006FE0 !important]    [color=#999999 !important]# first 3 digits
r[color=#DD1144 !important]'[-\s.]?'[color=#006FE0 !important] [color=#999999 !important]# followed by '-' or '.' or space
r[color=#DD1144 !important]'\d{4}$'[color=#006FE0 !important] [color=#999999 !important]# last 4 digits

让我们把它放到一个代码片段里：

import[color=#006FE0 !important] [color=teal !important]re

[color=teal !important]numbers[color=#006FE0 !important] = [color=#333333 !important][[color=#006FE0 !important] [color=#DD1144 !important]"123 555 6789"[color=#333333 !important],
[color=#006FE0 !important]          [color=#DD1144 !important]"1-(123)-555-6789"[color=#333333 !important],
[color=#006FE0 !important]          [color=#DD1144 !important]"(123-555-6789"[color=#333333 !important],
[color=#006FE0 !important]          [color=#DD1144 !important]"(123).555.6789"[color=#333333 !important],
[color=#006FE0 !important]          [color=#DD1144 !important]"123 55 6789"[color=#006FE0 !important] [color=#333333 !important]]

for[color=#006FE0 !important] [color=teal !important]number in[color=#006FE0 !important] [color=teal !important]numbers[color=#006FE0 !important]:
[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]match[color=#333333 !important](r[color=#DD1144 !important]'^'
[color=#006FE0 !important]                r[color=#DD1144 !important]'(1[-\s.])?'[color=#006FE0 !important]          [color=#999999 !important]# optional '1-', '1.' or '1'
[color=#006FE0 !important]                r[color=#DD1144 !important]'([color=#333333 !important]$)?'[color=#006FE0 !important]             [color=#999999 !important]# optional opening parenthesis
[color=#006FE0 !important]                r[color=#DD1144 !important]'\d{3}'[color=#006FE0 !important]             [color=#999999 !important]# the area code
[color=#006FE0 !important]                r[color=#DD1144 !important]'(?(2)$)'[color=#006FE0 !important]          [color=#999999 !important]# if there was opening parenthesis, close it
[color=#006FE0 !important]                r[color=#DD1144 !important]'[-\s.]?'[color=#006FE0 !important]             [color=#999999 !important]# followed by '-' or '.' or space
[color=#006FE0 !important]                r[color=#DD1144 !important]'\d{3}'[color=#006FE0 !important]             [color=#999999 !important]# first 3 digits
[color=#006FE0 !important]                r[color=#DD1144 !important]'[-\s.]?'[color=#006FE0 !important]             [color=#999999 !important]# followed by '-' or '.' or space
[color=#006FE0 !important]                r[color=#DD1144 !important]'\d{4}$\s*'[color=#333333 !important],[color=#002D7A !important]number[color=#333333 !important])[color=#006FE0 !important] [color=#999999 !important]# last 4 digits

[color=#006FE0 !important] if[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'{0} is valid'[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]number[color=#333333 !important])
[color=#006FE0 !important] else[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'{0} is not valid'[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]number[color=#333333 !important])

输出，不带空格：

[color=#009999 !important]123[color=#006FE0 !important] [color=#009999 !important]555[color=#006FE0 !important] [color=#009999 !important]6789[color=#006FE0 !important] is[color=#006FE0 !important] valid
[color=#009999 !important]1[color=#006FE0 !important]-[color=#333333 !important]([color=#009999 !important]123[color=#333333 !important])[color=#006FE0 !important]-[color=#009999 !important]555[color=#006FE0 !important]-[color=#009999 !important]6789[color=#006FE0 !important] is[color=#006FE0 !important] [color=teal !important]valid
[color=#333333 !important]([color=#009999 !important]123[color=#006FE0 !important]-[color=#009999 !important]555[color=#006FE0 !important]-[color=#009999 !important]6789[color=#006FE0 !important] is[color=#006FE0 !important] not[color=#006FE0 !important] [color=teal !important]valid
[color=#333333 !important]([color=#009999 !important]123[color=#333333 !important]).[color=#009999 !important]555.6789[color=#006FE0 !important] is[color=#006FE0 !important] valid
[color=#009999 !important]123[color=#006FE0 !important] [color=#009999 !important]55[color=#006FE0 !important] [color=#009999 !important]6789[color=#006FE0 !important] is[color=#006FE0 !important] not[color=#006FE0 !important] [color=#002D7A !important]valid

正则表达式是 python 的一个很好的功能，但是调试它们很艰难，而且正则表达式很容易就出错。

幸运的是，python 可以通过对 re.compile 或 re.match 设置 re.DEBUG (实际上就是整数 128) 标志就可以输出正则表达式的解析树。

import[color=#006FE0 !important] [color=teal !important]re

[color=teal !important]numbers[color=#006FE0 !important] = [color=#333333 !important][[color=#006FE0 !important] [color=#DD1144 !important]"123 555 6789"[color=#333333 !important],
[color=#006FE0 !important]          [color=#DD1144 !important]"1-(123)-555-6789"[color=#333333 !important],
[color=#006FE0 !important]          [color=#DD1144 !important]"(123-555-6789"[color=#333333 !important],
[color=#006FE0 !important]          [color=#DD1144 !important]"(123).555.6789"[color=#333333 !important],
[color=#006FE0 !important]          [color=#DD1144 !important]"123 55 6789"[color=#006FE0 !important] [color=#333333 !important]]

for[color=#006FE0 !important] [color=teal !important]number in[color=#006FE0 !important] [color=teal !important]numbers[color=#006FE0 !important]:
[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]match[color=#333333 !important](r[color=#DD1144 !important]'^'
[color=#006FE0 !important]                   r[color=#DD1144 !important]'(1[-\s.])?'[color=#006FE0 !important]       [color=#999999 !important]# optional '1-', '1.' or '1'
[color=#006FE0 !important]                   r[color=#DD1144 !important]'([color=#333333 !important]$)?'[color=#006FE0 !important]          [color=#999999 !important]# optional opening parenthesis
[color=#006FE0 !important]                   r[color=#DD1144 !important]'\d{3}'[color=#006FE0 !important]          [color=#999999 !important]# the area code
[color=#006FE0 !important]                   r[color=#DD1144 !important]'(?(2)$)'[color=#006FE0 !important]       [color=#999999 !important]# if there was opening parenthesis, close it
[color=#006FE0 !important]                   r[color=#DD1144 !important]'[-\s.]?'[color=#006FE0 !important]          [color=#999999 !important]# followed by '-' or '.' or space
[color=#006FE0 !important]                   r[color=#DD1144 !important]'\d{3}'[color=#006FE0 !important]          [color=#999999 !important]# first 3 digits
[color=#006FE0 !important]                   r[color=#DD1144 !important]'[-\s.]?'[color=#006FE0 !important]          [color=#999999 !important]# followed by '-' or '.' or space
[color=#006FE0 !important]                   r[color=#DD1144 !important]'\d{4}$'[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]number[color=#333333 !important],[color=#006FE0 !important] [color=teal !important]re[color=#333333 !important].[color=#002D7A !important]DEBUG[color=#333333 !important])[color=#006FE0 !important]  [color=#999999 !important]# last 4 digits

[color=#006FE0 !important] if[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'{0} is valid'[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]number[color=#333333 !important])
[color=#006FE0 !important] else[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'{0} is not valid'[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]number[color=#333333 !important])

解析树

[color=teal !important]at_beginning
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]1
[color=#006FE0 !important]  subpattern[color=#006FE0 !important] [color=#009999 !important]1
[color=#006FE0 !important] literal[color=#006FE0 !important] [color=#009999 !important]49
[color=#006FE0 !important] in
[color=#006FE0 !important]    literal[color=#006FE0 !important] [color=#009999 !important]45
[color=#006FE0 !important]    [color=teal !important]category category_space
[color=teal !important]    literal[color=#006FE0 !important] [color=#009999 !important]46
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]2147483648
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_space
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]1
[color=#006FE0 !important]  subpattern[color=#006FE0 !important] [color=#009999 !important]2
[color=#006FE0 !important] literal[color=#006FE0 !important] [color=#009999 !important]40
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]2147483648
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_space
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]3[color=#006FE0 !important] [color=#009999 !important]3
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_digit
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]2147483648
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_space
[color=teal !important]subpattern [color=#800080 !important]None
[color=#006FE0 !important]  [color=#002D7A !important]groupref[color=#333333 !important]_exists[color=#006FE0 !important] [color=#009999 !important]2
[color=#006FE0 !important] literal[color=#006FE0 !important] [color=#009999 !important]41
[color=#800080 !important]None
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]2147483648
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_space
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]1
[color=#006FE0 !important]  in
[color=#006FE0 !important] literal[color=#006FE0 !important] [color=#009999 !important]45
[color=#006FE0 !important] [color=teal !important]category category_space
[color=teal !important] literal[color=#006FE0 !important] [color=#009999 !important]46
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]2147483648
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_space
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]3[color=#006FE0 !important] [color=#009999 !important]3
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_digit
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]2147483648
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_space
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]1
[color=#006FE0 !important]  in
[color=#006FE0 !important] literal[color=#006FE0 !important] [color=#009999 !important]45
[color=#006FE0 !important] [color=teal !important]category category_space
[color=teal !important] literal[color=#006FE0 !important] [color=#009999 !important]46
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]2147483648
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_space
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]4[color=#006FE0 !important] [color=#009999 !important]4
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category category_digit
[color=teal !important]at at_end
[color=#002D7A !important]max[color=#333333 !important]_repeat[color=#006FE0 !important] [color=#009999 !important]0[color=#006FE0 !important] [color=#009999 !important]2147483648
[color=#006FE0 !important]  in
[color=#006FE0 !important] [color=teal !important]category [color=#002D7A !important]category[color=#333333 !important]_space
[color=#009999 !important]123[color=#006FE0 !important] [color=#009999 !important]555[color=#006FE0 !important] [color=#009999 !important]6789[color=#006FE0 !important] is[color=#006FE0 !important] valid
[color=#009999 !important]1[color=#006FE0 !important]-[color=#333333 !important]([color=#009999 !important]123[color=#333333 !important])[color=#006FE0 !important]-[color=#009999 !important]555[color=#006FE0 !important]-[color=#009999 !important]6789[color=#006FE0 !important] is[color=#006FE0 !important] [color=teal !important]valid
[color=#333333 !important]([color=#009999 !important]123[color=#006FE0 !important]-[color=#009999 !important]555[color=#006FE0 !important]-[color=#009999 !important]6789[color=#006FE0 !important] is[color=#006FE0 !important] not[color=#006FE0 !important] [color=teal !important]valid
[color=#333333 !important]([color=#009999 !important]123[color=#333333 !important]).[color=#009999 !important]555.6789[color=#006FE0 !important] is[color=#006FE0 !important] valid
[color=#009999 !important]123[color=#006FE0 !important] [color=#009999 !important]55[color=#006FE0 !important] [color=#009999 !important]6789[color=#006FE0 !important] is[color=#006FE0 !important] not[color=#006FE0 !important] [color=#002D7A !important]valid

贪婪和非贪婪

在我解释这个概念之前，我想先展示一个例子。我们要从一段 html 文本寻找锚标签：

import[color=#006FE0 !important] [color=teal !important]re

[color=#002D7A !important]html[color=#006FE0 !important] = [color=#DD1144 !important]'Hello <a href="http://pypix.com" title="pypix">Pypix</a>'

[color=#002D7A !important]m[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]findall[color=#333333 !important]([color=#DD1144 !important]'<a.*>.*<\/a>'[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]html[color=#333333 !important])
if[color=#006FE0 !important] [color=#002D7A !important]m[color=#006FE0 !important]:
[color=#006FE0 !important] [color=teal !important]print[color=#006FE0 !important] [color=#002D7A !important]m

结果将在意料之中：

['<a href="http://pypix.com" title="pypix">Pypix</a>']

我们改下输入，添加第二个锚标签：

import[color=#006FE0 !important] [color=teal !important]re

[color=#002D7A !important]html[color=#006FE0 !important] = [color=#DD1144 !important]'Hello <a href="http://pypix.com" title="pypix">Pypix</a>'[color=#006FE0 !important] [color=#333333 !important]\
[color=#006FE0 !important] [color=#DD1144 !important]'Hello <a href="http://example.com" title"example">Example</a>'

[color=#002D7A !important]m[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]findall[color=#333333 !important]([color=#DD1144 !important]'<a.*>.*<\/a>'[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]html[color=#333333 !important])
if[color=#006FE0 !important] [color=#002D7A !important]m[color=#006FE0 !important]:
[color=#006FE0 !important] [color=teal !important]print[color=#006FE0 !important] [color=#002D7A !important]m

结果看起来再次对了。但是不要上当了！如果我们在同一行遇到两个锚标签后，它将不再正确工作：

['<a href="http://pypix.com" title="pypix">Pypix</a>Hello <a href="http://example.com" title"example">Example</a>']

这次模式匹配了第一个开标签和最后一个闭标签以及在它们之间的所有的内容，成了一个匹配而不是两个单独的匹配。这是因为默认的匹配模式是“贪婪的”。

当处于贪婪模式时，量词(比如 * 和 +)匹配尽可能多的字符。

当你加一个问号在后面时（.*?）它将变为“非贪婪的”。

import[color=#006FE0 !important] [color=teal !important]re

[color=#002D7A !important]html[color=#006FE0 !important] = [color=#DD1144 !important]'Hello <a href="http://pypix.com" title="pypix">Pypix</a>'[color=#006FE0 !important] [color=#333333 !important]\
[color=#006FE0 !important] [color=#DD1144 !important]'Hello <a href="http://example.com" title"example">Example</a>'

[color=#002D7A !important]m[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]findall[color=#333333 !important]([color=#DD1144 !important]'<a.*?>.*?<\/a>'[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]html[color=#333333 !important])
if[color=#006FE0 !important] [color=#002D7A !important]m[color=#006FE0 !important]:
[color=#006FE0 !important] [color=teal !important]print[color=#006FE0 !important] [color=#002D7A !important]m

现在结果是正确的。

['<a href="http://pypix.com" title="pypix">Pypix</a>', '<a href="http://example.com" title"example">Example</a>']

前向界定符和后向界定符

一个前向界定符搜索当前的匹配之后搜索匹配。通过一个例子比较好解释一点。

下面的模式首先匹配 foo，然后检测是否接着匹配 bar：

import[color=#006FE0 !important] [color=teal !important]re

[color=#002D7A !important]strings[color=#006FE0 !important] = [color=#333333 !important][[color=#006FE0 !important]  [color=#DD1144 !important]"hello foo"[color=#333333 !important],[color=#006FE0 !important]       [color=#999999 !important]# returns False
[color=#006FE0 !important]          [color=#DD1144 !important]"hello foobar"[color=#006FE0 !important]  [color=#333333 !important]][color=#006FE0 !important] [color=#999999 !important]# returns True

for[color=#006FE0 !important] [color=teal !important]string[color=#006FE0 !important] in[color=#006FE0 !important] [color=#002D7A !important]strings[color=#006FE0 !important]:
[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]search[color=#333333 !important](r[color=#DD1144 !important]'foo(?=bar)'[color=#333333 !important],[color=#006FE0 !important] [color=teal !important]string[color=#333333 !important])
[color=#006FE0 !important] if[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'True'
[color=#006FE0 !important] else[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'False'

这看起来似乎没什么用，因为我们可以直接检测 foobar 不是更简单么。然而，它也可以用来前向否定界定。下面的例子匹配foo，当且仅当它的后面没有跟着 bar。

import[color=#006FE0 !important] [color=teal !important]re

[color=#002D7A !important]strings[color=#006FE0 !important] = [color=#333333 !important][[color=#006FE0 !important]  [color=#DD1144 !important]"hello foo"[color=#333333 !important],[color=#006FE0 !important]       [color=#999999 !important]# returns True
[color=#006FE0 !important]          [color=#DD1144 !important]"hello foobar"[color=#333333 !important],[color=#006FE0 !important]    [color=#999999 !important]# returns False
[color=#006FE0 !important]          [color=#DD1144 !important]"hello foobaz"[color=#333333 !important]][color=#006FE0 !important]    [color=#999999 !important]# returns True

for[color=#006FE0 !important] [color=teal !important]string[color=#006FE0 !important] in[color=#006FE0 !important] [color=#002D7A !important]strings[color=#006FE0 !important]:
[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]search[color=#333333 !important](r[color=#DD1144 !important]'foo(?!bar)'[color=#333333 !important],[color=#006FE0 !important] [color=teal !important]string[color=#333333 !important])
[color=#006FE0 !important] if[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'True'
[color=#006FE0 !important] else[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'False'

后向界定符类似，但是它查看当前匹配的前面的模式。你可以使用 (?> 来表示肯定界定，(?<! 表示否定界定。

下面的模式匹配一个不是跟在 foo 后面的 bar。

import[color=#006FE0 !important] [color=teal !important]re

[color=#002D7A !important]strings[color=#006FE0 !important] = [color=#333333 !important][[color=#006FE0 !important]  [color=#DD1144 !important]"hello bar"[color=#333333 !important],[color=#006FE0 !important]       [color=#999999 !important]# returns True
[color=#006FE0 !important]          [color=#DD1144 !important]"hello foobar"[color=#333333 !important],[color=#006FE0 !important]    [color=#999999 !important]# returns False
[color=#006FE0 !important]          [color=#DD1144 !important]"hello bazbar"[color=#333333 !important]][color=#006FE0 !important]    [color=#999999 !important]# returns True

for[color=#006FE0 !important] [color=teal !important]string[color=#006FE0 !important] in[color=#006FE0 !important] [color=#002D7A !important]strings[color=#006FE0 !important]:
[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]search[color=#333333 !important](r[color=#DD1144 !important]'(?<!foo)bar'[color=#333333 !important],[color=teal !important]string[color=#333333 !important])
[color=#006FE0 !important] if[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'True'
[color=#006FE0 !important] else[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'False'

条件(IF-Then-Else)模式

正则表达式提供了条件检测的功能。格式如下：

(?(?=regex)then|else)

条件可以是一个数字。表示引用前面捕捉到的分组。

比如我们可以用这个正则表达式来检测打开和闭合的尖括号：

import[color=#006FE0 !important] [color=teal !important]re

[color=#002D7A !important]strings[color=#006FE0 !important] = [color=#333333 !important][[color=#006FE0 !important]  [color=#DD1144 !important]"<pypix>"[color=#333333 !important],[color=#006FE0 !important] [color=#999999 !important]# returns true
[color=#006FE0 !important]          [color=#DD1144 !important]"<foo"[color=#333333 !important],[color=#006FE0 !important]    [color=#999999 !important]# returns false
[color=#006FE0 !important]          [color=#DD1144 !important]"bar>"[color=#333333 !important],[color=#006FE0 !important]    [color=#999999 !important]# returns false
[color=#006FE0 !important]          [color=#DD1144 !important]"hello"[color=#006FE0 !important] [color=#333333 !important]][color=#006FE0 !important]    [color=#999999 !important]# returns true

for[color=#006FE0 !important] [color=teal !important]string[color=#006FE0 !important] in[color=#006FE0 !important] [color=#002D7A !important]strings[color=#006FE0 !important]:
[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]search[color=#333333 !important](r[color=#DD1144 !important]'^(<)?[a-z]+(?(1)>)$'[color=#333333 !important],[color=#006FE0 !important] [color=teal !important]string[color=#333333 !important])
[color=#006FE0 !important] if[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'True'
[color=#006FE0 !important] else[color=#006FE0 !important]:
[color=#006FE0 !important]       [color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]'False'

在上面的例子中，1 表示分组 (<)，当然也可以为空因为后面跟着一个问号。当且仅当条件成立时它才匹配关闭的尖括号。

条件也可以是界定符。

无捕获组

分组，由圆括号括起来，将会捕获到一个数组，然后在后面要用的时候可以被引用。但是我们也可以不捕获它们。

我们先看一个非常简单的例子：

import[color=#006FE0 !important] [color=teal !important]re[color=#006FE0 !important]
[color=teal !important]string[color=#006FE0 !important] = [color=#DD1144 !important]'Hello foobar'[color=#006FE0 !important]
[color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]search[color=#333333 !important](r[color=#DD1144 !important]'(f.*)(b.*)'[color=#333333 !important],[color=#006FE0 !important] [color=teal !important]string[color=#333333 !important])[color=#006FE0 !important]
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"f* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#009999 !important]1[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints f* => foo
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"b* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#009999 !important]2[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints b* => bar

现在我们改动一点点，在前面加上另外一个分组 (H.*)：

import[color=#006FE0 !important] [color=teal !important]re[color=#006FE0 !important]
[color=teal !important]string[color=#006FE0 !important] = [color=#DD1144 !important]'Hello foobar'[color=#006FE0 !important]
[color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]search[color=#333333 !important](r[color=#DD1144 !important]'(H.*)(f.*)(b.*)'[color=#333333 !important],[color=#006FE0 !important] [color=teal !important]string[color=#333333 !important])[color=#006FE0 !important]
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"f* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#009999 !important]1[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints f* => Hello
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"b* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#009999 !important]2[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints b* => bar

模式数组改变了，取决于我们在代码中怎么使用这些变量，这可能会使我们的脚本不能正常工作。现在我们不得不找到代码中每一处出现了模式数组的地方，然后相应地调整下标。如果我们真的对一个新添加的分组的内容没兴趣的话，我们可以使它“不被捕获”，就像这样：

import[color=#006FE0 !important] [color=teal !important]re[color=#006FE0 !important]
[color=teal !important]string[color=#006FE0 !important] = [color=#DD1144 !important]'Hello foobar'[color=#006FE0 !important]
[color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]search[color=#333333 !important](r[color=#DD1144 !important]'(?:H.*)(f.*)(b.*)'[color=#333333 !important],[color=#006FE0 !important] [color=teal !important]string[color=#333333 !important])[color=#006FE0 !important]
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"f* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#009999 !important]1[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints f* => foo
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"b* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#009999 !important]2[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints b* => bar

通过在分组的前面添加 ?:，我们就再也不用在模式数组中捕获它了。所以数组中其他的值也不需要移动。

命名组

像前面那个例子一样，这又是一个防止我们掉进陷阱的方法。我们实际上可以给分组命名，然后我们就可以通过名字来引用它们，而不再需要使用数组下标。格式是：(?Ppattern) 我们可以重写前面那个例子，就像这样：

import[color=#006FE0 !important] [color=teal !important]re[color=#006FE0 !important]
[color=teal !important]string[color=#006FE0 !important] = [color=#DD1144 !important]'Hello foobar'[color=#006FE0 !important]
[color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]search[color=#333333 !important](r[color=#DD1144 !important]'(?P<fstar>f.*)(?P<bstar>b.*)'[color=#333333 !important],[color=#006FE0 !important] [color=teal !important]string[color=#333333 !important])[color=#006FE0 !important]
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"f* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#DD1144 !important]'fstar'[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints f* => foo
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"b* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#DD1144 !important]'bstar'[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints b* => bar

现在我们可以添加另外一个分组了，而不会影响模式数组里其他的已存在的组：

import[color=#006FE0 !important] [color=teal !important]re[color=#006FE0 !important]
[color=teal !important]string[color=#006FE0 !important] = [color=#DD1144 !important]'Hello foobar'[color=#006FE0 !important]
[color=#002D7A !important]pattern[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]search[color=#333333 !important](r[color=#DD1144 !important]'(?P<hi>H.*)(?P<fstar>f.*)(?P<bstar>b.*)'[color=#333333 !important],[color=#006FE0 !important] [color=teal !important]string[color=#333333 !important])[color=#006FE0 !important]
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"f* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#DD1144 !important]'fstar'[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints f* => foo
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"b* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#DD1144 !important]'bstar'[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints b* => bar
[color=teal !important]print[color=#006FE0 !important] [color=#DD1144 !important]"h* => {0}"[color=#333333 !important].[color=teal !important]format[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important].[color=teal !important]group[color=#333333 !important]([color=#DD1144 !important]'hi'[color=#333333 !important]))[color=#006FE0 !important] [color=#999999 !important]# prints b* => Hello

使用回调函数

在 Python 中 re.sub() 可以用来给正则表达式替换添加回调函数。

让我们来看看这个例子，这是一个 e-mail 模板：

import[color=#006FE0 !important] [color=teal !important]re[color=#006FE0 !important]
[color=#002D7A !important]template[color=#006FE0 !important] = [color=#DD1144 !important]"Hello [first_name] [last_name], \
[color=#DD1144 !important] Thank you for purchasing [product_name] from [store_name]. \
[color=#DD1144 !important] The total cost of your purchase was [product_price] plus [ship_price] for shipping. \
[color=#DD1144 !important] You can expect your product to arrive in [ship_days_min] to [ship_days_max] business days. \
[color=#DD1144 !important] Sincerely, \
[color=#DD1144 !important] [store_manager_name]"[color=#006FE0 !important]
[color=#999999 !important]# assume dic has all the replacement data
[color=#999999 !important]# such as dic['first_name'] dic['product_price'] etc...
[color=#002D7A !important]dic[color=#006FE0 !important] = [color=#333333 !important]{[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"first_name"[color=#006FE0 !important] : [color=#DD1144 !important]"John"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"last_name"[color=#006FE0 !important] : [color=#DD1144 !important]"Doe"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"product_name"[color=#006FE0 !important] : [color=#DD1144 !important]"iphone"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"store_name"[color=#006FE0 !important] : [color=#DD1144 !important]"Walkers"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"product_price"[color=#006FE0 !important]: [color=#DD1144 !important]"$500"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"ship_price"[color=#006FE0 !important]: [color=#DD1144 !important]"$10"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"ship_days_min"[color=#006FE0 !important]: [color=#DD1144 !important]"1"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"ship_days_max"[color=#006FE0 !important]: [color=#DD1144 !important]"5"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"store_manager_name"[color=#006FE0 !important]: [color=#DD1144 !important]"DoeJohn"[color=#006FE0 !important]
[color=#333333 !important]}[color=#006FE0 !important]
[color=#002D7A !important]result[color=#006FE0 !important] = [color=teal !important]re[color=#333333 !important].[color=teal !important]compile[color=#333333 !important](r[color=#DD1144 !important]'\[(.*)\]'[color=#333333 !important])[color=#006FE0 !important]
[color=teal !important]print[color=#006FE0 !important] [color=#002D7A !important]result[color=#333333 !important].[color=teal !important]sub[color=#333333 !important]([color=#DD1144 !important]'John'[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]template[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]count[color=#006FE0 !important]=[color=#009999 !important]1[color=#333333 !important])

注意到每一个替换都有一个共同点，它们都是由一对中括号括起来的。我们可以用一个单独的正则表达式来捕获它们，并且用一个回调函数来处理具体的替换。

所以用回调函数是一个更好的办法：

import[color=#006FE0 !important] [color=teal !important]re[color=#006FE0 !important]
[color=#002D7A !important]template[color=#006FE0 !important] = [color=#DD1144 !important]"Hello [first_name] [last_name], \
[color=#DD1144 !important] Thank you for purchasing [product_name] from [store_name]. \
[color=#DD1144 !important] The total cost of your purchase was [product_price] plus [ship_price] for shipping. \
[color=#DD1144 !important] You can expect your product to arrive in [ship_days_min] to [ship_days_max] business days. \
[color=#DD1144 !important] Sincerely, \
[color=#DD1144 !important] [store_manager_name]"[color=#006FE0 !important]
[color=#999999 !important]# assume dic has all the replacement data
[color=#999999 !important]# such as dic['first_name'] dic['product_price'] etc...
[color=#002D7A !important]dic[color=#006FE0 !important] = [color=#333333 !important]{[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"first_name"[color=#006FE0 !important] : [color=#DD1144 !important]"John"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"last_name"[color=#006FE0 !important] : [color=#DD1144 !important]"Doe"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"product_name"[color=#006FE0 !important] : [color=#DD1144 !important]"iphone"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"store_name"[color=#006FE0 !important] : [color=#DD1144 !important]"Walkers"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"product_price"[color=#006FE0 !important]: [color=#DD1144 !important]"$500"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"ship_price"[color=#006FE0 !important]: [color=#DD1144 !important]"$10"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"ship_days_min"[color=#006FE0 !important]: [color=#DD1144 !important]"1"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"ship_days_max"[color=#006FE0 !important]: [color=#DD1144 !important]"5"[color=#333333 !important],[color=#006FE0 !important]
[color=#006FE0 !important] [color=#DD1144 !important]"store_manager_name"[color=#006FE0 !important]: [color=#DD1144 !important]"DoeJohn"[color=#006FE0 !important]
[color=#333333 !important]}[color=#006FE0 !important]
def[color=#006FE0 !important] [color=teal !important]multiple_replace[color=#333333 !important]([color=#002D7A !important]dic[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]text[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important] [color=#002D7A !important]pattern[color=#006FE0 !important] = [color=#DD1144 !important]"|"[color=#333333 !important].[color=teal !important]join[color=#333333 !important]([color=teal !important]map[color=#333333 !important](lambda[color=#006FE0 !important] [color=#002D7A !important]key[color=#006FE0 !important] : [color=teal !important]re[color=#333333 !important].[color=teal !important]escape[color=#333333 !important]([color=#DD1144 !important]"["[color=#006FE0 !important]+[color=#002D7A !important]key[color=#006FE0 !important]+[color=#DD1144 !important]"]"[color=#333333 !important]),[color=#006FE0 !important] [color=#002D7A !important]dic[color=#333333 !important].[color=teal !important]keys[color=#333333 !important]()))
[color=#006FE0 !important] return[color=#006FE0 !important] [color=teal !important]re[color=#333333 !important].[color=teal !important]sub[color=#333333 !important]([color=#002D7A !important]pattern[color=#333333 !important],[color=#006FE0 !important] lambda[color=#006FE0 !important] [color=#002D7A !important]m[color=#006FE0 !important]: [color=#002D7A !important]dic[color=#333333 !important][[color=#002D7A !important]m[color=#333333 !important].[color=teal !important]group[color=#333333 !important]()[[color=#009999 !important]1[color=#006FE0 !important]:-[color=#009999 !important]1[color=#333333 !important]]],[color=#006FE0 !important] [color=#002D7A !important]text[color=#333333 !important])[color=#006FE0 !important]
[color=teal !important]print[color=#006FE0 !important] [color=teal !important]multiple_replace[color=#333333 !important]([color=#002D7A !important]dic[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]template[color=#333333 !important])

不要重复发明轮子

更重要的可能是知道在什么时候不要使用正则表达式。在许多情况下你都可以找到替代的工具。

解析 [X]HTML

Stackoverflow 上的一个答案用一个绝妙的解释告诉了我们为什么不应该用正则表达式来解析 [X]HTML。

你应该使用使用 HTML 解析器，Python 有很多选择：

ElementTree 是标准库的一部分
BeautifulSoup 是一个流行的第三方库
lxml 是一个功能齐全基于 c 的快速的库

后面两个即使是处理畸形的 HTML 也能很优雅，这给大量的丑陋站点带来了福音。

ElementTree 的一个例子：

from[color=#006FE0 !important] [color=teal !important]xml[color=#333333 !important].[color=teal !important]etree import[color=#006FE0 !important] [color=teal !important]ElementTree
[color=#002D7A !important]tree[color=#006FE0 !important] = [color=#002D7A !important]ElementTree[color=#333333 !important].[color=teal !important]parse[color=#333333 !important]([color=#DD1144 !important]'filename.html'[color=#333333 !important])[color=#006FE0 !important]
for[color=#006FE0 !important] [color=teal !important]element in[color=#006FE0 !important] [color=#002D7A !important]tree[color=#333333 !important].[color=teal !important]findall[color=#333333 !important]([color=#DD1144 !important]'h1'[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important] [color=teal !important]print[color=#006FE0 !important] [color=#002D7A !important]ElementTree[color=#333333 !important].[color=teal !important]tostring[color=#333333 !important]([color=#002D7A !important]element[color=#333333 !important])

其他

在使用正则表达式之前，这里有很多其他可以考虑的工具。

感谢阅读！

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