Ask for help on using re

[jak]

Il 07/08/2021 11:18, jak ha scritto:
> Il 07/08/2021 04:23, Jach Feng ha scritto:
>> jak 在 2021年8月6日 星期五下午4:10:05 [UTC+8] 的信中寫道：
>>> Il 05/08/2021 11:40, Jach Feng ha scritto:
>>>> I want to distinguish between numbers with/without a dot attached:
>>>>
>>>>>>> text = 'ch 1. is\nch 23. is\nch 4 is\nch 56 is\n'
>>>>>>> re.compile(r'ch \d{1,}[.]').findall(text)
>>>> ['ch 1.', 'ch 23.']
>>>>>>> re.compile(r'ch \d{1,}[^.]').findall(text)
>>>> ['ch 23', 'ch 4 ', 'ch 56 ']
>>>>
>>>> I can guess why the 'ch 23' appears in the second list. But how to 
>>>> get rid of it?
>>>>
>>>> --Jach
>>>>
>>> import re
>>> t = 'ch 1. is\nch 23. is\nch 4 is\nch 56 is\n'
>>> r = re.compile(r'(ch +\d+\.)|(ch +\d+)', re.M)
>>>
>>> res = r.findall(t)
>>>
>>> dot = [x[1] for x in res if x[1] != '']
>>> udot = [x[0] for x in res if x[0] != '']
>>>
>>> print(f"dot: {dot}")
>>> print(f"undot: {udot}")
>>>
>>> out:
>>>
>>> dot: ['ch 4', 'ch 56']
>>> undot: ['ch 1.', 'ch 23.']
>> The result can be influenced by the order of re patterns?
>>
>>>>> import re
>>>>> t = 'ch 1. is\nch 23. is\nch 4 is\nch 56 is\n'
>>>>> re.compile(r'(ch +\d+\.)|(ch +\d+)', re.M).findall(t)
>> [('ch 1.', ''), ('ch 23.', ''), ('', 'ch 4'), ('', 'ch 56')]
>>
>>>>> re.compile(r'(ch +\d+)|(ch +\d+\.)', re.M).findall(t)
>> [('ch 1', ''), ('ch 23', ''), ('ch 4', ''), ('ch 56', '')]
>>
>> --Jach
>>
> Yes, when the patterns intersect each other as in your case. the
> difference between the 2 patterns is the "." in addition. The logical or
> does not continue checking when the condition is satisfied, so it is a
> good idea, in these cases, to search for the most complete patterns
> before the others.
> 
> 
PS
... the behavior of the logical or that I have described is not typical
of regular expressions but it is common in all programming languages.