Python正则加flag后匹配不到句首

问题描述

问 题

Python2.7

这样写是正常的，可以匹配到：

pattern = re.compile(u'wechat', re.I)
pattern.search(u'wechat online')

这样写就匹配不到了：

pattern = re.compile(u'wechat')
pattern.search(u'wechat online', re.I)

flag的位置很影响吗？

这个情况只是句首会出现，其他位置貌似没影响……

解决方案

楼主,好好看文档啊
你的p是一个RegexObject,如下

class re.RegexObject 
The RegexObject class supports the following methods and attributes:

search(string[, pos[, endpos]]) 
Scan through string looking for a location where this regular expression produces a match, and return a corresponding MatchObject instance. Return None if no position in the string matches the pattern; note that this is different from finding a zero-length match at some point in the string.

The optional second parameter pos gives an index in the string where the search is to start; it defaults to 0. This is not completely equivalent to slicing the string; the '^' pattern character matches at the real beginning of the string and at positions just after a newline, but not necessarily at the index where the search is to start.

The optional parameter endpos limits how far the string will be searched; it will be as if the string is endpos characters long, so only the characters from pos to endpos - 1 will be searched for a match. If endpos is less than pos, no match will be found, otherwise, if rx is a compiled regular expression object, rx.search(string, 0, 50) is equivalent to rx.search(string[:50], 0).

>>> pattern = re.compile("d")
>>> pattern.search("dog")     # Match at index 0
<_sre.SRE_Match object at ...>
>>> pattern.search("dog", 1)  # No match; search doesn't include the "d"

search方法根本没有一个叫做flags的参数!

re.compile的文档也帮你摘抄下吧

The module defines several functions, constants, and an exception. Some of the functions are simplified versions of the full featured methods for compiled regular expressions. Most non-trivial applications always use the compiled form.

re.compile(pattern, flags=0) 
Compile a regular expression pattern into a regular expression object, which can be used for matching using its match() and search() methods, described below.

The expression’s behaviour can be modified by specifying a flags value. Values can be any of the following variables, combined using bitwise OR (the | operator).

The sequence

prog = re.compile(pattern)
result = prog.match(string)
is equivalent to

result = re.match(pattern, string)
but using re.compile() and saving the resulting regular expression object for reuse is more efficient when the expression will be used several times in a single program.

Note
The compiled versions of the most recent patterns passed to re.match(), re.search() or re.compile() are cached, so programs that use only a few regular expressions at a time needn’t worry about compiling regular expressions.

耐心读文档

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