Python之re模块一

1.正则表达式主要作用于字符串，对字符串进行模糊匹配

2.

. 除换行符意\n以外的任意字符，如需匹配换行符，可以在匹配模式中加入re.S

* 零到正无穷个

+ 一到正无穷个

? [0,1]

\d [0-9]

\w 字母数字下划线

^ 字符串的开头

$ 字符串的结尾

[] 字符集

[^] 非字符集里的字符串匹配

() 分组优先  (?:)取消分组优先级

[]字符集的特殊符号只有3个：

-：表示范围

^：表示非

\：表示转义

除以上元字符有特殊意义外其余的元字符在字符集中都是普通字符

 (?P<group_name>) (?P=group_name) 给分组数据起组名，后面的分组规则必须和前面的一种

正则表达式使用反斜杠" \ "来代表特殊形式或用作转义字符，这里跟Python的语法冲突，因此，Python用" \\\\ "表示正则表达式中的" \ "，因为正则表达式中如果要匹配" \ "，需要用\来转义，变成" \\ "，而Python语法中又需要对字符串中每一个\进行转义，所以就变成了" \\\\ "。 

为了使正则表达式具有更好的可读性，Python特别设计了原始字符串(raw string)。raw string就是用'r'作为字符串的前缀，如 r"\n"：表示两个字符"\"和"n"，而不是换行符了。Python中写正则表达式时推荐使用这种形式。

个人总结：?在贪婪匹配中不起作用：有可能是零个，也有可能是1个，如果在贪婪匹配中量词后面加上?，匹配时就会按照最少的个数进行匹配

贴出源码中对各规则的解释：

r"""Support for regular expressions (RE).This module provides regular expression matching operations similar tothose found in Perl.  It supports both 8-bit and Unicode strings; boththe pattern and the strings being processed can contain null bytes andcharacters outside the US ASCII range.Regular expressions can contain both special and ordinary characters.Most ordinary characters, like "A", "a", or "0", are the simplestregular expressions; they simply match themselves.  You canconcatenate ordinary characters, so last matches the string 'last'.The special characters are:    "."      Matches any character except a newline.    "^"      Matches the start of the string.    "$"      Matches the end of the string or just before the newline at             the end of the string.    "*"      Matches 0 or more (greedy) repetitions of the preceding RE.             Greedy means that it will match as many repetitions as possible.    "+"      Matches 1 or more (greedy) repetitions of the preceding RE.    "?"      Matches 0 or 1 (greedy) of the preceding RE.    *?,+?,?? Non-greedy versions of the previous three special characters.    {m,n}    Matches from m to n repetitions of the preceding RE.    {m,n}?   Non-greedy version of the above.    "\\"     Either escapes special characters or signals a special sequence.    []       Indicates a set of characters.             A "^" as the first character indicates a complementing set.    "|"      A|B, creates an RE that will match either A or B.    (...)    Matches the RE inside the parentheses.             The contents can be retrieved or matched later in the string.    (?aiLmsux) Set the A, I, L, M, S, U, or X flag for the RE (see below).    (?:...)  Non-grouping version of regular parentheses.    (?P<name>...) The substring matched by the group is accessible by name.    (?P=name)     Matches the text matched earlier by the group named name.    (?#...)  A comment; ignored.    (?=...)  Matches if ... matches next, but doesn't consume the string.    (?!...)  Matches if ... doesn't match next.    (?<=...) Matches if preceded by ... (must be fixed length).    (?<!...) Matches if not preceded by ... (must be fixed length).    (?(id/name)yes|no) Matches yes pattern if the group with id/name matched,                       the (optional) no pattern otherwise.The special sequences consist of "\\" and a character from the listbelow.  If the ordinary character is not on the list, then theresulting RE will match the second character.    \number  Matches the contents of the group of the same number.    \A       Matches only at the start of the string.    \Z       Matches only at the end of the string.    \b       Matches the empty string, but only at the start or end of a word.    \B       Matches the empty string, but not at the start or end of a word.    \d       Matches any decimal digit; equivalent to the set [0-9] in             bytes patterns or string patterns with the ASCII flag.             In string patterns without the ASCII flag, it will match the whole             range of Unicode digits.    \D       Matches any non-digit character; equivalent to [^\d].    \s       Matches any whitespace character; equivalent to [ \t\n\r\f\v] in             bytes patterns or string patterns with the ASCII flag.             In string patterns without the ASCII flag, it will match the whole             range of Unicode whitespace characters.    \S       Matches any non-whitespace character; equivalent to [^\s].    \w       Matches any alphanumeric character; equivalent to [a-zA-Z0-9_]             in bytes patterns or string patterns with the ASCII flag.             In string patterns without the ASCII flag, it will match the             range of Unicode alphanumeric characters (letters plus digits             plus underscore).             With LOCALE, it will match the set [0-9_] plus characters defined             as letters for the current locale.    \W       Matches the complement of \w.    \\       Matches a literal backslash.This module exports the following functions:    match     Match a regular expression pattern to the beginning of a string.    fullmatch Match a regular expression pattern to all 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.    compile   Compile a pattern into a RegexObject.    purge     Clear the regular expression cache.    escape    Backslash all non-alphanumerics in a string.Some of the functions in this module takes flags as optional parameters:    A  ASCII       For string patterns, make \w, \W, \b, \B, \d, \D                   match the corresponding ASCII character categories                   (rather than the whole Unicode categories, which is the                   default).                   For bytes patterns, this flag is the only available                   behaviour and needn't be specified.    I  IGNORECASE  Perform case-insensitive matching.    L  LOCALE      Make \w, \W, \b, \B, dependent on the current locale.    M  MULTILINE   "^" matches the beginning of lines (after a newline)                   as well as the string.                   "$" matches the end of lines (before a newline) as well                   as the end of the string.    S  DOTALL      "." matches any character at all, including the newline.    X  VERBOSE     Ignore whitespace and comments for nicer looking RE's.    U  UNICODE     For compatibility only. Ignored for string patterns (it                   is the default), and forbidden for bytes patterns.This module also defines an exception 'error'."""