常用的功能函数包括：match、search、findall、sub
1、re.match()函数
函数语法：
re.match(pattern, string, flags=0)
1 def match(pattern, string, flags=0):
2 """Try to apply the pattern at the start of the string, returning
3 a match object, or None if no match was found."""
4 return _compile(pattern, flags).match(string)
函数参数说明：
pattern：匹配的正则表达式
string：要匹配的字符串
flag：标志位，用于控制正则表达式的匹配方式（是否匹配大小写、多行匹配等）
作用：match()函数只在字符串的开始位置尝试匹配正则表达式，即从位置0开始匹配。如果匹配成功，则返回一个匹配的对象；如果字符串开始不符合正则表达式，则匹配失败，函数返回None。
1 import re
2 test = 'http://news.163.com/17/0624/10/CNMHVBJP0001899N.html'
3 print(re.match(r'http',test)) # <_sre.SRE_Match object; span=(0, 4), match='http'>
4 print(re.match(r'news',test)) # None
2、re.search()函数
函数语法：
1 re.search(pattern, string[, flags])
1 def search(pattern, string, flags=0):
2 """Scan through string looking for a match to the pattern, returning
3 a match object, or None if no match was found."""
4 return _compile(pattern, flags).search(string)
re.search()匹配整个字符串，直到找到第一个匹配的，如果字符串中没有匹配的，则返回None。
1 import re
2 test = 'I am a loving child to learn.'
3 print(re.search(r'I',test)) # <_sre.SRE_Match object; span=(0, 1), match='I'>
4 print(re.search(r'learn',test)) # <_sre.SRE_Match object; span=(23, 28), match='learn'>
5 print(re.search(r'alina',test)) # None
3、re.sub()函数
函数语法：
1 re.sub(pattern,repl,string,count,flags)
1 def sub(pattern, repl, string, count=0, flags=0):
2 """Return the string obtained by replacing the leftmost
3 non-overlapping occurrences of the pattern in string by the
4 replacement repl. repl can be either a string or a callable;
5 if a string, backslash escapes in it are processed. If it is
6 a callable, it's passed the match object and must return
7 a replacement string to be used."""
8 return _compile(pattern, flags).sub(repl, string, count)
函数参数说明：
pattern：匹配的正则表达式
repl：替换的字符串
String：要被查找替换的原始字符串
count：匹配后替换的最大次数，默认0表示途欢所有的匹配
re.sub()函数用于替换字符串中的匹配项。
1 import re
2 test = 'I am a loving child to learn.'
3 print(re.sub(r'child','MMMMM',test)) # 替换字符串，将child 替换成MMMMM
4、re.findall()函数
函数语法：
1 re.findall(pattern,string,flags)
1 def findall(pattern, string, flags=0):
2 """Return a list of all non-overlapping matches in the string.
3
4 If one or more capturing groups are present in the pattern, return
5 a list of groups; this will be a list of tuples if the pattern
6 has more than one group.
7
8 Empty matches are included in the result."""
9 return _compile(pattern, flags).findall(string)
re.findall()可以获取字符串中所有匹配的字符串
1 import re
2 test = '<a href="http://www.educity.cn/zhibo/" target="_blank">直播课堂</a>'
3 print(re.findall(r'<a href="(.*)" target="_blank">(.*)</a>',test)) #[('http://www.educity.cn/zhibo/', '直播课堂')]
