Python re module (regular expressions)

regular expressions (RE) 简介

　　re模块是python中处理正在表达式的一个模块

 r"""Support for regular expressions (RE).

 This module provides regular expression matching operations similar to
 those found in Perl.  It supports both 8-bit and Unicode strings; both
 the pattern and the strings being processed can contain null bytes and
 characters outside the US ASCII range.

 Regular expressions can contain both special and ordinary characters.
 Most ordinary characters, like "A", "a", or "0", are the simplest
 regular expressions; they simply match themselves.  You can
 concatenate 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 list
 below.  If the ordinary character is not on the list, then the
 resulting 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'.

 """

虽然在Python 中使用正则表达式有几个步骤，但每一步都相当简单。
1．用import re 导入正则表达式模块。
2．用re.compile()函数创建一个Regex 对象（记得使用原始字符串）。
3．向Regex 对象的search()方法传入想查找的字符串。它返回一个Match 对象。
4．调用Match 对象的group()方法，返回实际匹配文本的字符串。

              向re.compile()传递原始字符串
Python 中转义字符使用倒斜杠（\）。字符串'\n'表示一个换行字符，
而不是倒斜杠加上一个小写的n。你需要输入转义字符\\，才能打印出一个倒斜杠。
所以'\\n'表示一个倒斜杠加上一个小写的n。但是，通过在字符串的第一个引号之
前加上r，可以将该字符串标记为原始字符串，它不包括转义字符。
因为正则表达式常常使用倒斜杠，向re.compile()函数传入原始字符串就很方
便， 而不是输入额外得到斜杠。
输入r'\d\d\d-\d\d\d-\d\d\d\d' ，
比输入'\\d\\d\\d-\\d\\d\\d-\\d\\d\\d\\d'要容易得多。

1.compile()

• python代码最终会被编译为字节码，之后才被解释器执行。
• 在模式匹配之前，正在表达式模式必须先被编译成regex对象，预先编译可以提高性能，re.compile()就是用于提供此功能。

def compile(pattern, flags=0):
    "Compile a regular expression pattern, returning a pattern object."
    return _compile(pattern, flags)

2. findall(pattern, string, flags=0)

• match和search均用于匹配单值，即：只能匹配字符串中的一个，如果想要匹配到字符串中所有符合条件的元素，则需要使用 findall。
• findall，获取非重复的匹配列表；

1. 如果有一个组则以列表形式返回，且每一个匹配均是字符串；
2. 如果模型中有多个组，则以列表形式返回，且每一个匹配均是元祖；空的匹配也会包含在结果中

def findall(pattern, string, flags=0):
    """Return a list of all non-overlapping matches in the string.

    If one or more capturing groups are present in the pattern, return
    a list of groups; this will be a list of tuples if the pattern
    has more than one group.

    Empty matches are included in the result."""
    return _compile(pattern, flags).findall(string)

3. match(pattern, string, flags=0)

　　从字符串的开头进行匹配， 匹配成功就返回一个匹配对象，匹配失败就返回None

　　flags的几种值:

1. X 忽略空格和注释
2. I 忽略大小写的区别 case-insensitive matching
3. S . 匹配任意字符，包括新行

def match(pattern, string, flags=0):
    """Try to apply the pattern at the start of the string, returning
    a match object, or None if no match was found."""
    return _compile(pattern, flags).match(string)

4. search(pattern, string, flags=0)

　　浏览整个字符串去匹配第一个，未匹配成功返回None

def search(pattern, string, flags=0):
    """Scan through string looking for a match to the pattern, returning
    a match object, or None if no match was found."""
    return _compile(pattern, flags).search(string)

• search() vs. match()

# Python offers two different primitive operations based on regular expressions:
# re.match() checks for a match only at the beginning of the string,
# while re.search() checks for a match anywhere in the string (this is what Perl does by default).

>>> re.match("c", "abcdef")    # No match
>>> re.search("c", "abcdef")   # Match
<_sre.SRE_Match object at ...>

# Regular expressions beginning with '^' can be used with search() to restrict the match at the beginning of the string:
# re.match('str', "string")  等价于  re.search('^str', "string")
>>> re.match("c", "abcdef")    # No match
>>> re.search("^c", "abcdef")  # No match
>>> re.search("^a", "abcdef")  # Match
<_sre.SRE_Match object at ...>

# Note however that in MULTILINE mode match() only matches at the beginning of the string,
# whereas using search() with a regular expression beginning with '^' will match at the beginning of each line.
# 多行匹配 模式 对 match() 无效
# 带^的正则匹配 search() 在 多行匹配 模式下，会去 字符串的每一行 匹配 要查找的字符或字符串

>>> re.match('X', 'A\nB\nX', re.MULTILINE)  # No match
>>> re.search('^X', 'A\nB\nX', re.MULTILINE)  # Match
<_sre.SRE_Match object at ...>

5. sub(pattern,repl,string,count=0,flags=0)

　　替换匹配成功的指定位置字符串

def sub(pattern, repl, string, count=0, flags=0):
    """Return the string obtained by replacing the leftmost
    non-overlapping occurrences of the pattern in string by the
    replacement repl.  repl can be either a string or a callable;
    if a string, backslash escapes in it are processed.  If it is
    a callable, it's passed the match object and must return
    a replacement string to be used."""
    return _compile(pattern, flags).sub(repl, string, count)

6. split(pattern,string,maxsplit=0,flags=0)
　　根据正则匹配分割字符串

def split(pattern, string, maxsplit=0, flags=0):
    """Split the source string by the occurrences of the pattern,
    returning a list containing the resulting substrings.  If
    capturing parentheses are used in pattern, then the text of all
    groups in the pattern are also returned as part of the resulting
    list.  If maxsplit is nonzero, at most maxsplit splits occur,
    and the remainder of the string is returned as the final element
    of the list."""
    return _compile(pattern, flags).split(string, maxsplit)

7. group()与groups()

匹配对象的两个主要方法：

group()   返回所有匹配对象，或返回某个特定子组，如果没有子组，返回全部匹配对象

groups() 返回一个包含唯一或所有子组的的元组，如果没有子组，返回空元组