python3 regex .* vs .*? : match more or less, match, search, findall, sub

1.  .* vs  .*? :

import re

input='helloworld, 123abc,test'

pattern1='.*(\d+)'

pattern2='.*?(\d+)'

result1=re.match(pattern1, input)

result2=re.match(pattern2, input)

print(result1.group(1)) 

=> 3    # .* match more, \d+ match 3 only, 

print(result2.group(1)) 

=> 123 # .*? match less, \d+ match 123.  Generally used.

2. match, search, findall

pattern='test'

input2=' a test, b test '

m=re.match(pattern, input2)

print(m)

=>None

m=re.search(pattern, input2)

print(m)

=><_sre.SRE_Match object; span=(2, 6), match='test'>

print(m.group())

=>test

m=re.findall(pattern, input2)

print(m)

=>['test', 'test']

3. replace by sub() 

html = '''    <ul id="list" class="list-group">

        <li data-view="5"><a href="null">test1</a></li>

        <li data-view="6"><a href="null">test2</a></li>

    </ul>

　　　　'''

html = re.sub('<a.*?>|</a>', '', html)

print(html)

=>    <ul id="list" class="list-group">

        <li data-view="5">test1</li>

        <li data-view="6">test2</li>

 </ul>

pattern = '<li.*?>(.*?)</li>'

results = re.findall(pattern, html, re.S)  # re.S改行

print(results)=> ["test1","test2"]

pattern = '<li.*>(.*)</li>'

results = re.findall(pattern, html, re.S)  # re.S改行

print(results)=> ["test2"]

4. get match group by name 

pattern = '<li.*?>(?P<text>.*?)</li>'

results = re.seach(pattern, html, re.S)  # re.S改行

print(results.group(1))=>test1

print(results.group("text"))=>test1

5. match lookahead assertion. (?=, ?! )

input1='hello world!'

input2='hello goodbye!'

pattern1='hello (?=world)'       #?=world: lookahead match

pattern2='hello (?!word)'  #?!world: lookahead not match 

r1=re.pattern(pattern1, input1)

print(r1.group())=>hello

r2=re.pattern(pattern2, input1)

print(r2.group())=>None

r3=re.pattern(pattern2, input2)

print(r3.group())=>hello

6. match lookbehind assertion. (?<=,?<! )

>>> m = re.search('(?<=abc)def', 'abcdef')  #?<=lookbehind match
>>> m.group(0)
'def'

m = re.search(r'(?<=-)\w+', 'spam-egg')     # must has - before \w+  
>>> m.group(0)
'egg'