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Python模块之re模块

Python模块之re模块

 

在谈re模块之前,我们先来谈谈什么是正则表达式

正则表达式(Regular Expression)通常被用来检索、替换那些符合某个模式(规则)的文本。

此处的Regular即是规则、规律的意思,Regular Expression即“描述某种规则的表达式”之意。

 

那python是通过什么来支持正则表达式的呢,就是今天我们要说的re模块。

Python通过re模块支持正则表达式,re 模块使 Python 语言拥有全部的正则表达式功能。

 

我们先来介绍一下正则的一些语法:

 

1.规则:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 2.优先级:

 

 

 

3.常用的函数:

同样,先用 help() 方法来看看re都有点什么:

Help on module re:

NAME
    re - Support for regular expressions (RE).

MODULE REFERENCE
    https://docs.python.org/3.8/library/re
    
    The following documentation is automatically generated from the Python
    source files.  It may be incomplete, incorrect or include features that
    are considered implementation detail and may vary between Python
    implementations.  When in doubt, consult the module reference at the
    location listed above.

DESCRIPTION
    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 Pattern object.
        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'.

CLASSES
    builtins.Exception(builtins.BaseException)
        error
    builtins.object
        Match
        Pattern
    
    class Match(builtins.object)
     |  The result of re.match() and re.search().
     |  Match objects always have a boolean value of True.
     |  
     |  Methods defined here:
     |  
     |  __copy__(self, /)
     |  
     |  __deepcopy__(self, memo, /)
     |  
     |  __getitem__(self, key, /)
     |      Return self[key].
     |  
     |  __repr__(self, /)
     |      Return repr(self).
     |  
     |  end(self, group=0, /)
     |      Return index of the end of the substring matched by group.
     |  
     |  expand(self, /, template)
     |      Return the string obtained by doing backslash substitution on the string template, as done by the sub() method.
     |  
     |  group(...)
     |      group([group1, ...]) -> str or tuple.
     |      Return subgroup(s) of the match by indices or names.
     |      For 0 returns the entire match.
     |  
     |  groupdict(self, /, default=None)
     |      Return a dictionary containing all the named subgroups of the match, keyed by the subgroup name.
     |      
     |      default
     |        Is used for groups that did not participate in the match.
     |  
     |  groups(self, /, default=None)
     |      Return a tuple containing all the subgroups of the match, from 1.
     |      
     |      default
     |        Is used for groups that did not participate in the match.
     |  
     |  span(self, group=0, /)
     |      For match object m, return the 2-tuple (m.start(group), m.end(group)).
     |  
     |  start(self, group=0, /)
     |      Return index of the start of the substring matched by group.
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors defined here:
     |  
     |  endpos
     |      The index into the string beyond which the RE engine will not go.
     |  
     |  lastgroup
     |      The name of the last matched capturing group.
     |  
     |  lastindex
     |      The integer index of the last matched capturing group.
     |  
     |  pos
     |      The index into the string at which the RE engine started looking for a match.
     |  
     |  re
     |      The regular expression object.
     |  
     |  regs
     |  
     |  string
     |      The string passed to match() or search().
    
    class Pattern(builtins.object)
     |  Compiled regular expression object.
     |  
     |  Methods defined here:
     |  
     |  __copy__(self, /)
     |  
     |  __deepcopy__(self, memo, /)
     |  
     |  __eq__(self, value, /)
     |      Return self==value.
     |  
     |  __ge__(self, value, /)
     |      Return self>=value.
     |  
     |  __gt__(self, value, /)
     |      Return self>value.
     |  
     |  __hash__(self, /)
     |      Return hash(self).
     |  
     |  __le__(self, value, /)
     |      Return self<=value.
     |  
     |  __lt__(self, value, /)
     |      Return self<value.
     |  
     |  __ne__(self, value, /)
     |      Return self!=value.
     |  
     |  __repr__(self, /)
     |      Return repr(self).
     |  
     |  findall(self, /, string, pos=0, endpos=9223372036854775807)
     |      Return a list of all non-overlapping matches of pattern in string.
     |  
     |  finditer(self, /, string, pos=0, endpos=9223372036854775807)
     |      Return an iterator over all non-overlapping matches for the RE pattern in string.
     |      
     |      For each match, the iterator returns a match object.
     |  
     |  fullmatch(self, /, string, pos=0, endpos=9223372036854775807)
     |      Matches against all of the string.
     |  
     |  match(self, /, string, pos=0, endpos=9223372036854775807)
     |      Matches zero or more characters at the beginning of the string.
     |  
     |  scanner(self, /, string, pos=0, endpos=9223372036854775807)
     |  
     |  search(self, /, string, pos=0, endpos=9223372036854775807)
     |      Scan through string looking for a match, and return a corresponding match object instance.
     |      
     |      Return None if no position in the string matches.
     |  
     |  split(self, /, string, maxsplit=0)
     |      Split string by the occurrences of pattern.
     |  
     |  sub(self, /, repl, string, count=0)
     |      Return the string obtained by replacing the leftmost non-overlapping occurrences of pattern in string by the replacement repl.
     |  
     |  subn(self, /, repl, string, count=0)
     |      Return the tuple (new_string, number_of_subs_made) found by replacing the leftmost non-overlapping occurrences of pattern with the replacement repl.
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors defined here:
     |  
     |  flags
     |      The regex matching flags.
     |  
     |  groupindex
     |      A dictionary mapping group names to group numbers.
     |  
     |  groups
     |      The number of capturing groups in the pattern.
     |  
     |  pattern
     |      The pattern string from which the RE object was compiled.
    
    class error(builtins.Exception)
     |  error(msg, pattern=None, pos=None)
     |  
     |  Exception raised for invalid regular expressions.
     |  
     |  Attributes:
     |  
     |      msg: The unformatted error message
     |      pattern: The regular expression pattern
     |      pos: The index in the pattern where compilation failed (may be None)
     |      lineno: The line corresponding to pos (may be None)
     |      colno: The column corresponding to pos (may be None)
     |  
     |  Method resolution order:
     |      error
     |      builtins.Exception
     |      builtins.BaseException
     |      builtins.object
     |  
     |  Methods defined here:
     |  
     |  __init__(self, msg, pattern=None, pos=None)
     |      Initialize self.  See help(type(self)) for accurate signature.
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors defined here:
     |  
     |  __weakref__
     |      list of weak references to the object (if defined)
     |  
     |  ----------------------------------------------------------------------
     |  Static methods inherited from builtins.Exception:
     |  
     |  __new__(*args, **kwargs) from builtins.type
     |      Create and return a new object.  See help(type) for accurate signature.
     |  
     |  ----------------------------------------------------------------------
     |  Methods inherited from builtins.BaseException:
     |  
     |  __delattr__(self, name, /)
     |      Implement delattr(self, name).
     |  
     |  __getattribute__(self, name, /)
     |      Return getattr(self, name).
     |  
     |  __reduce__(...)
     |      Helper for pickle.
     |  
     |  __repr__(self, /)
     |      Return repr(self).
     |  
     |  __setattr__(self, name, value, /)
     |      Implement setattr(self, name, value).
     |  
     |  __setstate__(...)
     |  
     |  __str__(self, /)
     |      Return str(self).
     |  
     |  with_traceback(...)
     |      Exception.with_traceback(tb) --
     |      set self.__traceback__ to tb and return self.
     |  
     |  ----------------------------------------------------------------------
     |  Data descriptors inherited from builtins.BaseException:
     |  
     |  __cause__
     |      exception cause
     |  
     |  __context__
     |      exception context
     |  
     |  __dict__
     |  
     |  __suppress_context__
     |  
     |  __traceback__
     |  
     |  args

FUNCTIONS
    compile(pattern, flags=0)
        Compile a regular expression pattern, returning a Pattern object.
    
    escape(pattern)
        Escape special characters in a string.
    
    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.
    
    finditer(pattern, string, flags=0)
        Return an iterator over all non-overlapping matches in the
        string.  For each match, the iterator returns a Match object.
        
        Empty matches are included in the result.
    
    fullmatch(pattern, string, flags=0)
        Try to apply the pattern to all of the string, returning
        a Match object, or None if no match was found.
    
    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.
    
    purge()
        Clear the regular expression caches
    
    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.
    
    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.
    
    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.
    
    subn(pattern, repl, string, count=0, flags=0)
        Return a 2-tuple containing (new_string, number).
        new_string is the string obtained by replacing the leftmost
        non-overlapping occurrences of the pattern in the source
        string by the replacement repl.  number is the number of
        substitutions that were made. 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.
    
    template(pattern, flags=0)
        Compile a template pattern, returning a Pattern object

DATA
    A = re.ASCII
    ASCII = re.ASCII
    DOTALL = re.DOTALL
    I = re.IGNORECASE
    IGNORECASE = re.IGNORECASE
    L = re.LOCALE
    LOCALE = re.LOCALE
    M = re.MULTILINE
    MULTILINE = re.MULTILINE
    S = re.DOTALL
    U = re.UNICODE
    UNICODE = re.UNICODE
    VERBOSE = re.VERBOSE
    X = re.VERBOSE
    __all__ = ['match', 'fullmatch', 'search', 'sub', 'subn', 'split', 'fi...

VERSION
    2.2.1

FILE
    e:\program files (x86)\python_3.8\lib\re.py


None
View Code

 

官方文档在这:https://docs.python.org/3.8/library/re

 

 

 

1. 函数 re.compile ( )

re.compile(pattern,flags = 0) 将正则表达式的字符串形式编译成正则表达式对象,该对象可以使用match()等 方法进行操作

import re
test="Hi, nice to meet you where are you from?" #需要被匹配的字符串
k=re.compile(r'\w*o\w*') #将匹配带o的字符串编译成一个正则对象
print (k.findall(test)) #显示所有包含o的字符串
OUT:['to', 'you', 'you', 'from']

 

2. 函数 re.match ( )

re.match(pattern, string, flags=0) 如果字符串开头处有匹配模式的子字符串,则返回匹配对象。

即使在多行 模式下,也只匹配字符串开头,而非每行开头

import re
match = re.match(r'dog', 'dog cat dog')
print(match.group())
OUT:dog

 

3. 函数 re.search ( )

re.search(pattern, string, flags=0) search() 函数和 match()函数近似, 但是search()函数不局限于查询匹配字 符串开头。

import re
search = re.search(r'cat', 'dog cat dog cat')
search.group(0)
OUT:cat

 

4. 函数 re.findall ( )

re.findall(pattern, string, flags=0) 比起返回匹配的内容,findall()会给我们返回一个匹配内容的列表

import re
find_dog = re.findall(r'dog', 'dog cat dog')
find_cat = re.findall(r'cat', 'dog cat dog')
print (find_dog, find_cat)
OUT:['dog', 'dog'] ['cat']

 

5. 函数 re.finditer ( )

re.finditer(pattern, string, flags=0) 搜索string,返回一个顺序访问每一个匹配结果(Match对象)的迭代器。

import re
matches = re.finditer(r'([a-zA-Z]+) \d+', 'June 24, August 9, Dec 12') for match in matches: print ("Match at index: %s, %s" % (match.start(), match.end()))
OUT:  Match at index: 0, 7
       Match at index: 9, 17
       Match at index: 19, 25

 

6. 函数 re.split ( )

re.split(pattern,string,maxsplit=0,flags=0) 按照能够匹配的子串将string分割后返回列表。maxsplit用于指定最大分割次数,不指定将全部分割。

import re
p = re.compile(r'(\d+)')
q =
re.compile(r'\d+')

print(p.split('one111112two245568three3873984723four4'))
print(q.split('one111112two245568three3873984723four4'))
OUT:['one', '111112', 'two', '245568', 'three', '3873984723', 'four', '4', '']
   ['one', 'two', 'three', 'four', '']

注意:

import re
num = re.split(r'-', '0376-2233-445') # 以-分割字符串,返回列表中不包含
num1 = re.split(r'(-)','0376-2233-445') # 以(-)分组分割字符串,返回列表中包含
num2 = re.split(r'-','0376-2233-445',maxsplit = 1) # 指定分割次数为1,剩下的子串作为一个整体
num3 = re.split(r'(\d+)','0376-2233-445') # 分组从字符串开头匹配,列表首尾为一个空字符串
print('only numebr: ', num)
print('numebr and sign: ', num1)
print('numebr split once: ', num2)
print('split with space: ', num3)
OUT:only numebr:  ['0376', '2233', '445']
    numebr and sign:  ['0376', '-', '2233', '-', '445']
    numebr split once:  ['0376', '2233-445']
    split with space:  ['', '0376', '-', '2233', '-', '445', '']

 

7. 函数 re.sub( )

re.sub(pattern, string, flags=0) sub()函数会返回按要求修改过的字符串内容。

import re
phone = "2004-959-559 # This is Phone Number"
# 删除字符串中python的注释
num = re.sub(r'#.*$', "", phone)
print ("Phone Num : "+ num)
# 删除除数字以外的所有元素
num = re.sub(r'\D', "", phone)
print ("Phone Num : "+ num)
OUT:Phone Num : 2004-959-559 
    Phone Num : 2004959559 

 

到这,常见的函数我们就介绍完了。

 

不过,这里要注意两个函数的使用:

re.compile 用于编译正则表达式,生成一个正则表达式( Pattern )对象;

re.findall 用于在字符串中找到正则表达式所匹配的所有子串,并返回一个列表,如果没有找到匹配的,则返回空列表。

 

最后:

我们可以利用上述知识来简单写一个计算器:

简单要求如下:

  1. 实现加减乘除及拓号优先级解析
  2. 用户输入 1 - 2 * ( (60-30 +(-40/5) * (9-2*5/3 + 7 /3*99/4*2998 +10 * 568/14 )) - (-4*3)/ (16-3*2) )等类似公式后,必须自己解析里面的(),+,-,*,/符号和公式,运算后得出结果,结果必须与真实的计算器所得出的结果一致

参考代码:https://www.cnblogs.com/guguguhuha/p/14423853.html

 

发表于 2021-02-27 10:10  guguguhuha  阅读(0)  评论(0编辑  收藏
 

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