Python標準庫 - re
對於正則表達式, 有些字符具有特殊含義, 需使用反斜杠字符'\'轉義, 使其表示本身含義. 如想匹配字符'\', 卻要寫成'\\\\', 很是困擾. Python中Raw string解決了該問題, 只需給'\'加上前綴'r'即可, 如r'\n', 表示'\'和'n'兩個普通字符, 而不是原來的換行. 前綴'r'類似於sed命令的-r(use extended regular expressions)參數.
正則表達式可包括兩部分, 一是正常字符, 表本身含義; 二是特殊字符, 表一類正常字符, 或字符數量...
re模塊提供了諸多方法進行正則匹配.
match Match a regular expression pattern to the beginning of a string.
search Search a string for the presence of a pattern.
sub Substitute occurrences of a pattern found in a string.
subn Same as sub, but also return the number of substitutions made.
split Split a string by the occurrences of a pattern.
findall Find all occurrences of a pattern in a string.
finditer Return an iterator yielding a match object for each match.
purge Clear the regular expression cache.
escape Backslash all non-alphanumerics in a string.
還有compile函數, 其較特殊, 將匹配模式編譯為一個正則表達式對象(RegexObject, _sre.SRE_Pattern), 並返回, 該對象仍然可以使用上述這些函數. 這也從側面說明了, 對於re模塊, 有非編譯和編譯兩種使用方式, 如下所示.
1.
result = re.match(pattern, string)
2.
prog = re.compile(pattern)
result = prog.match(string)
它們達到的效果是相同的, 只是後者暫存了正則表達式對象, 對於某塊代碼中頻繁使用該正則表達式的情形, 後者性能一般會高於前者.
對於match()和search()匹配成功, 會返回一個匹配對象(Match Object, _sre.SRE_Match), 其也有若幹方法, 下面幾個較常用.
group
group([group1, ...]) -> str or tuple.
Return subgroup(s) of the match by indices or names.
For 0 returns the entire match.
groups(...)
groups([default=None]) -> tuple.
Return a tuple containing all the subgroups of the match, from 1.
The default argument is used for groups
that did not participate in the match
end(...)
end([group=0]) -> int.
Return index of the end of the substring matched by group.
start(...)
start([group=0]) -> int.
Return index of the start of the substring matched by group.
至此對re模塊框架性梳理就這樣了, 給出些例子, 對上面的內容總結下.
1.
In [23]: text = "He was carefully disguised but captured quickly by police."
In [24]: re.findall(r"\w+ly", text)
Out[24]: ['carefully', 'quickly']
2.
In [25]: m = re.match(r"(\w+) (\w+)", "Isaac Newton, physicist")
In [26]: m.group(0)
Out[26]: 'Isaac Newton'
In [27]: m.group(1)
Out[27]: 'Isaac'
In [28]: m.group(2)
Out[28]: 'Newton'
In [29]: m.group(1, 2)
Out[29]: ('Isaac', 'Newton')
3.
In [31]: account = "abcxyz_"
In [32]: replace_regex = re.compile(r'_$')
In [33]: replace_regex.sub(account[0], account)
Out[33]: 'abcxyza'
正則表達式使用中的細節還有很多, 這裏無法盡數, 實踐過程中慢慢體會和總結吧.
Python標準庫 - re