正則表達式(regular expression)描述了一種字符串匹配的模式（pattern），可以用來檢查一個串是否含有某種子串、將匹配的子串替換或者從某個串中取出符合某個條件的子串等。

Python 自1.5版本起增加了re 模塊，它提供 Perl 風格的正則表達式模式。

re 模塊使 Python 語言擁有全部的正則表達式功能。

compile 函數(shù)根據(jù)一個模式字符串和可選的標志參數(shù)生成一個正則表達式對象。該對象擁有一系列方法用于正則表達式匹配和替換。

本文重點給大家介紹python 正則表達式語法。

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...) 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).? ? (?

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.

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下面看下正則表達式匹配的流程：

正則表達式的大致匹配過程是：依次拿出表達式和文本中的字符比較，如果每一個字符都能匹配，則匹配成功；一旦有匹配不成功的字符則匹配失敗。如果表達式中有量詞或邊界，這個過程會稍微有一些不同，但也是很好理解的，自己多使用幾次就能明白。

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