[re regular expression] ------- PYTHON crawler basics 1

We often need to retrieve specific text that matches a certain pattern (rule) during the crawling process. At this time, re regular expressions can be used to help.
A regular expression is a logical formula for string operations. It is to use some defined specific characters and combinations of these specific characters to form a "regular string". This "regular string" is used to express the string A filtering logic for .

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[re regular expression] ------- PYTHON crawler basics 1

Dry goods for beginners

Online regular expression test link

Before the article starts, recommend an online regular expression test link (https://tool.oschina.net/regex)
online regular expression test link

re base expression

model	describe
^	matches the beginning of the string
$	matches the end of the string
*	matches 0 or more expressions
+	Match 1 or more expressions
.	Match any character, except newline, when the re.DOTALL flag is specified, it can match any character including newline
[…]	Used to represent a group of characters, listed separately: [amk] matches 'a', 'm' or 'k'
[^…]	Characters not in []: abc matches characters other than a, b, c
?	Match 0 or 1 fragment defined by the preceding regular expression, non-greedy
{n}	Matches exactly n preceding expressions. For example, o{2} would not match the "o" in "Bob", but would match both o's in "food"
{n,}	Matches n occurrences of the preceding expression. For example, o{2,} would not match the "o" in "Bob", but would match all o's in "foooood". "o{1,}" is equivalent to "o+". "o{0,}" is equivalent to "o*".
{n,m}	Match n to m times the segment defined by the preceding regular expression, greedily
a	b
(re)	matches an expression enclosed in parentheses, also denoting a group
(?imx)	Regular expressions contain three optional flags: i, m, or x. Only the area enclosed in parentheses is affected.
(?-imx)	The regular expression turns off the i, m, or x optional flags. Only the area enclosed in parentheses is affected.
(?: re)	like (…), but does not denote a group
(?imx: re)	Use i, m, or x optional flags in parentheses
(?-imx: re)	Do not use i, m, or x optional flags in parentheses
(?#…)	note.
(?= re)	Forward positive delimiter. If the contained regular expression, denoted by ... , succeeds if it matches successfully at the current position, fails otherwise. But once the contained expression has been tried, the matching engine does not advance at all; the remainder of the pattern also tries the right side of the delimiter.
(?! re)	Lookahead negation delimiter. The opposite of a positive delimiter; succeeds when the contained expression cannot be matched at the current position in the string
(?> re)	Independent pattern for matching, omitting backtracking.
\w	Matches alphanumerics and underscores
\W	Match non-alphanumeric and underscore
\s	Matches any whitespace character, equivalent to [\t\n\r\f].
\S	matches any non-null character
\d	Match any number, equivalent to [0-9].
\D	matches any non-digit
\A	match string starts
\Z	Match the end of the string, if there is a newline, only match the end string before the newline.
\z	match end of string
\G	Match where the last match was done.
\b	Matches a word boundary, that is, the position between a word and a space. For example, 'er\b' would match 'er' in "never", but not 'er' in "verb".
\B	Matches non-word boundaries. 'er\B' matches 'er' in 'verb', but not 'er' in 'never'.
\n, \t, etc.	Matches a newline character. Matches a tab character. wait
\1…\9	Matches the contents of the nth group.
\10	匹配第n个分组的内容，如果它经匹配。否则指的是八进制字符码的表达式。

正文

1.匹配函数 re.match

尝试从字符串的起始位置匹配一个模式,如果不是起始位置就匹配成功的话,match()就返回None。
语法：re.match(pattern,string,flags())

1.1 最常规的匹配

python3环境下输入：

import re
content = "Hello 123 4567 World_This is a Regex Demo"
print(len(content))
result = re.match('^Hello\s\d\d\d\s\d{4}\s\w{10}.*Demo$',content)  #匹配整个content
print(result)
print(result.group())  #匹配结果
print(result.span())   #匹配结果的范围

结果输出为：

41
<re.Match object; span=(0, 41), match='Hello 123 4567 World_This is a Regex Demo'>
Hello 123 4567 World_This is a Regex Demo
(0, 41)

知识点

1）len(content) ：计算content的长度，注意空格也算一个长度
2）匹配细节
^匹配字符串的开头
\s匹配任意空白字符，这里匹配空格
\d匹配任意数字，等价于 [0-9]；\d\d\d这里连续3个数字123；\d{4}匹配连续4个数字4567；
\w匹配字母数字及下划线，w{10}这里匹配World_This；
. 匹配任意字符，除了换行符。*匹配0个或多个的表达式。
.*这里匹配 is a Regex ；注意这里是包含空格的。
$：匹配字符串的末尾。

1.2 泛匹配

python3环境下输入：

import re
content = "Hello 123 4567 World_This is a Regex Demo"
result = re.match('^Hello.*Demo$',content)
print(result)
print(result.group())  
print(result.span())

结果输出为：

<re.Match object; span=(0, 41), match='Hello 123 4567 World_This is a Regex Demo'>
Hello 123 4567 World_This is a Regex Demo
(0, 41)

知识点

.*这里匹配 123 4567 World_This is a Regex

1.3 匹配目标(提取部分)

python3环境下输入：

import re
content = "Hello 1234567 World_This is a Regex Demo"
result = re.match('^Hello\s(\d+)\sWorld.*Demo$',content)
print(result)
print(result.group(1))  #匹配第一个括号内的结果
print(result.span())

结果输出为：

<re.Match object; span=(0, 40), match='Hello 1234567 World_This is a Regex Demo'>
1234567
(0, 40)

知识点

匹配1个或多个的表达式。\d+这里匹配1234567。
将提取部分加上括号，eg: (\d+)。对应地 result.group(1)) #返回第一个括号内的结果

1.4 贪婪匹配.*

python3环境下输入：

import re
content = "Hello 1234567 World_This is a Regex Demo"
result = re.match('^He.*(\d+).*Demo$',content)   # .*会匹配尽可能多的字符
print(result)
print(result.group(1)) 
print(result.span())

结果输出为：

<re.Match object; span=(0, 40), match='Hello 1234567 World_This is a Regex Demo'>
7
(0, 40)

知识点

贪婪匹配.*会匹配尽可能多的字符。
这里.*(\d+).*中第一个.*匹配到llo 123456，(\d+)匹配到7，第二个.*匹配到 World_This is a Regex 。

1.5 非贪婪匹配.*？

python3环境下输入：

import re
content = "Hello 1234567 World_This is a Regex Demo"
result = re.match('^He.*?(\d+).*Demo$',content)   # .*?会匹配尽可能少的字符
print(result)
print(result.group(1)) 
print(result.span())

结果输出为：

<re.Match object; span=(0, 40), match='Hello 1234567 World_This is a Regex Demo'>
1234567
(0, 40)

知识点

.*？会匹配尽可能少的字符

1.6 换行符

python3环境下输入：

import re
content = '''Hello 1234567 World_This
is a Regex Demo
'''
result = re.match('^He.*?(\d+).*Demo$',content)    #.*无法匹配换行符  
print(result)

结果输出为：

None

python3环境下输入：

import re
content = '''Hello 1234567 World_This
is a Regex Demo
'''
result = re.match('^He.*?(\d+).*Demo$',content,re.S)     #re.S 代替换行符 
print(result)
print(result.group(1))  
print(result.span())

结果输出为：

<re.Match object; span=(0, 40), match='Hello 1234567 World_This\nis a Regex Demo'>
1234567
(0, 40)

知识点

.*无法匹配换行符
re.S 代替换行符

1.7 转义(匹配特殊字符)？

python3环境下输入：

import re
content = 'price is $5.00'
result = re.match('price is $5.00',content)      #包含特殊字符不能直接全部输入
print(result)

结果输出为：

None

python3环境下输入：

import re
content = 'price is $5.00'
result = re.match('price is \$5\.00',content)       #给特殊字符加双斜杆即可
print(result)

结果输出为：

<re.Match object; span=(0, 14), match='price is $5.00'>

知识点

包含特殊字符不能直接全部输入
给特殊字符加双斜杆即可

总结
尽量使用泛匹配,使用括号得到匹配目标,尽量使用非贪婪模式,有换行符就用re.S

2.匹配函数 re.search

re.search 更推荐使用，扫描整个字符串并返回第一个成功的匹配

python3环境下输入：

import re
content = "Extra stings Hello 1234567 World_This is a Regex Demo Extra stings"
result = re.match('Hello.*?(\d+).*?Demo',content)   # re.match必须是从头开始匹配
print(result)

结果输出为：

None

python3环境下输入：

iimport re
content = "Extra stings Hello 1234567 World_This is a Regex Demo Extra stings"
result = re.search('Hello.*?(\d+).*?Demo',content)   # re.search不是必须从头开始匹配
print(result)

结果输出为：

<re.Match object; span=(13, 53), match='Hello 1234567 World_This is a Regex Demo'>

知识点

re.match必须是从头开始匹配
re.search不是必须从头开始匹配

python3环境下输入：

import re
html = '''*<div id="songs-list">
<h2 class=title">经典老歌</h2>
<p class="introduction">
经典老歌列表
</p>
<ul id="list" class"list -group">
<li data -view="2">一路上有你</li>
<li data -view="7" >
<a href="/2.mp3" singer="任贤齐">沧海声笑</a>
</li>
<li data -view="4" class="active">
<a href="/3.mp3" singer="齐秦">往事随风</a>
</li>
<ll data-view="6"><a href="*/4.mp3" singer="beyond">光辉岁月</a></li>
<li data _view="5"><a href-=*/5.mp3" singer="陈慧琳">记事本</a></li>
<li data -view="5">
<a href="/6.mp3" singer="邓丽君">但愿人长久</a>
</i>
</ul>
</div>'''
result = re.search('<li.*?active.*?singer="(.*?)">(.*?)</a>',html,re.S)   #!!!re.S很重要
if result:
    print(result.group(1),result.group(2))   ##  返回第一个满足条件的结果
result = re.search('<li.*?singer="(.*?)">(.*?)</a>',html,re.S)   #删掉.*?active
if result:
    print(result.group(1),result.group(2))

结果输出为：

齐秦 往事随风
任贤齐 沧海声笑

3.匹配函数 re.findall

re.findall 搜索字符串,以列表形式返回全部能匹配的字段

python3环境下输入：

import re
html = '''*<div id="songs-list">
<h2 class=title">经典老歌</h2>
<p class="introduction">
经典老歌列表
</p>
<ul id="list" class"list -group">
<li data -view="2">一路上有你</li>
<li data -view="7" >
<a href="/2.mp3" singer="任贤齐">沧海一声笑</a>
</li>
<li data -view="4" class="active">
<a href="/3.mp3" singer="齐秦">往事随风</a>
</li>
<ll data-view="6"><a href="*/4.mp3" singer="beyond">光辉岁月</a></li>
<li data _view="5"><a href-=*/5.mp3" singer="陈慧琳">记事本</a></li>
<li data -view="5">
<a href="/6.mp3" singer="邓丽君">但愿人长久</a>
</i>
</ul>
</div>'''
# results = re.findall('<li.*?href="(.*?)".*?singer="(.*?)">(.*?)</a>',html,re.S) 
# print(results)
# print(type(results))
# for result in results:
#     print(result)
#     print(result[0],result[1])     # 匹配到的是从第二个开始的
    
#如果想要把第一个"一路上有你"和其他歌名匹配到 
# #方法一：
# results = re.findall('<li.*?>\s*?(<a.*?>)?(\w+)(</a>)?\s*?</li>',html,re.S) 
# # \s*?  换行空白字符可能有可能没有   (<a.*?>)?  a标签可能有可能没有   (\w+)歌名字符串   （</a>）?  </a>可能有可能没有
# print(results)
# for result in results:
#     print(result[1]) 
    
#方法二：
html=re.sub("<a.*?>|</a>","",html)
# print(html)
results=re.findall('<li.*?>(.*?)</li>',html,re.S)
print(results)   #有的有换行符，有的没有
for result in results:
    print(result.strip())   #strip()把换行符去掉

结果输出为：

['一路上有你', '\n沧海一声笑\n', '\n往事随风\n', '记事本']
一路上有你
沧海一声笑
往事随风
记事本

4.匹配函数 re.sub

re.sub 替换字符串中每一个匹配的子串后返回替换后的字符串

python3环境下输入：

import re
content = "Extra stings Hello 1234567 World_This is a Regex Demo Extra stings"
result = re.sub('\d+','',content)   # 将数字部分变为空
print(result)

结果输出为：

Extra stings Hello  World_This is a Regex Demo Extra stings

python3环境下输入：

import re
content = "Extra stings Hello 1234567 World_This is a Regex Demo Extra stings"
result = re.sub('\d+','Replacement',content)   # 将数字部分变为替换字符串
print(result)

结果输出为：

Extra stings Hello Replacement World_This is a Regex Demo Extra stings

python3环境下输入：

import re
content = "Extra stings Hello 1234567 World_This is a Regex Demo Extra stings"
result = re.sub('(\d+)',r'\1 8910',content)   # 将数字部分继续添加,加括号,再r'\1 注意这里有空格,\1
print(result)

结果输出为：

Extra stings Hello 1234567 8910 World_This is a Regex Demo Extra stings

5.匹配函数 re.compile

re.compile 将正则字符串编译成正则表达式对象，以便于复用该匹配模式

python3环境下输入：

import re
content = '''Hello 1234567 World_This
is a Regex Demo
'''
pattern = re.compile('Hello.*Demo',re.S)     #re.S 代替换行符 
result = re.match(pattern,content)
print(result)
print(result.group())  
print(result.span())

结果输出为：

<re.Match object; span=(0, 40), match='Hello 1234567 World_This\nis a Regex Demo'>
Hello 1234567 World_This
is a Regex Demo
(0, 40)

6. 实战练习

练习一：提取网页发布时间
python3环境下输入：

import re
html="""<span class="info">
                选择字体：[<span class="set-font" data-font="lg">大</span>-<span class="set-font" data-font="md">中</span>-<span class="set-font" data-font="sm">小</span>]
              </span>
<span class="info">发布时间：2021-06-09 08:48:16</span>
<span class="info share-box" id="toShare">
"""
publishDate = re.findall('<span.*?class="info">发布时间：(.*?)</span>',html,re.S)
# publishDate = re.findall('<span.*?class="info.*?share-box".*?id="(.*?)">',html,re.S)
print(publishDate)

结果输出为：

['2021-06-09 08:48:16']

练习二：提取发件附件名称
python3环境下输入：

import re
html='''
<td align="center" style="padding:5px 10px;"><strong>发证附件：</strong></td>
<td style="padding:5px 10px;"><p><a href="/picture/lslzfl/picture/old/a78fb67a-e02f-4a58-9499-f0a847254660.jpg" style="font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; font-size: 16px; font-family: sans-serif; white-space: normal;" target="_blank">发证附件.pdf</a></p></td>
</tr>
</div>
'''
publishDate = re.findall('发证附件：.*?href="(.*?)"',html,re.S)
# publishDate = re.findall('<span.*?class="info.*?share-box".*?id="(.*?)">',html,re.S)
print(publishDate)

结果输出为：

['/picture/lslzfl/picture/old/a78fb67a-e02f-4a58-9499-f0a847254660.jpg']

练习二：提取土拍部分信息
python3环境下输入：

import re
html="""
<div class="land-name-title">

                    温岭市石桥头镇下宅吴村石桥头学工路北侧、振兴南路东侧地块                    <span class="tag-guipai">招标</span>
<span class="tag-deal tag-deal__wait">待成交</span>
</div>
"""
title = re.findall('<div.*?class="land-name-title">(.*?)<span',html,re.S)
aaa = re.findall('<span.*?class="tag-guipai">(.*?)</span>',html)
bbb = re.findall('<span.*?class="tag-deal.*?tag-deal__wait">(.*?)</span>',html)
print(title)
print(aaa)
print(bbb)

结果输出为：

['\n\n                    温岭市石桥头镇下宅吴村石桥头学工路北侧、振兴南路东侧地块                    ']
['招标']
['待成交']