如何使用 '？' 在python中两个匹配模式之间提取可选子字符串？

Question

I was answering this questio n. Consider this string

str1 = '{"show permission allowed to 16": "show permission to 16\\nSchool permissions from group 17:student to group 16:teacher:\\n\\tAllow ALL-00\\nSchool permissions from group 18:library to group 16(Temp):teacher:\\n\\tNo Allow ALL-00\\nSchool permissions from group 20:Gym to group 16:teacher:\\n\\tCheck ALL-00\\nRTYAHY: FALSE\\nRTYAHY: FALSE\\n\\n#"}'

and suppose I want to extract the number after each substring from group and the substring after \\\\t having the smallest matching string.

I did this with the following regular expression

import re
res = re.findall(r'from group (\d+).*?\\t(.*? ALL-..)', str1)

output is:

[('17', 'Allow ALL-00'), ('18', 'No Allow ALL-00'), ('20', 'Check ALL-00')]

Now between each substring I am extracting (the number and the substring after \\t ) there might be an optional substring, whose value is Temp I want to extract (if present). For example between 18 and No Allow ALL-00 there is the substring Temp I would like to extract.

I tried using ? as follows:

res = re.findall(r'from group (\d+).*?(Temp)?.*?\\t(.*? ALL-..)', str1)

but the corresponding second element of the resulting tuples is always empty:

[('17', '', 'Allow ALL-00'), ('18', '', 'No Allow ALL-00'), ('20', '', 'Check ALL-00')]

while I was expecting something like:

[('17', '', 'Allow ALL-00'), ('18', 'Temp', 'No Allow ALL-00'), ('20', '', 'Check ALL-00')]

How to extract substrings in this case? What is the mistake I am doing?

One further question: suppose I want my resulting list not having this element (the one containing Temp ): should I just use [^] and then the corresponding matching pattern?

Answer 1

The reason why it is not capturing Temp is because you have made it optional due to which .*? consumes it, and Temp does not get captured in your optional group.

To solve that problem, you can use negative look ahead to reject Temp getting captured except any other character using this regex,

from group (\d+)(?:(?!Temp).)*?(Temp)?(?:(?!Temp).)*?\\t(.*? ALL-..)
                   ^^^^^^^^^ This rejects Temp getting captured except any other character

Regex explanation:

• from group - literal matching of this text
• (?:(?!Temp).)*? - ?: means its a non-capturing group which by default is a capturing group and this means that capturing anything but stop when you see Temp string and * means capture zero or more characters. So this captures any string which doesn't contain Temp and ? means as less as possible
• (Temp)? - Optionally capture Temp if present
• (?:(?!Temp).)*? - Again capture any character zero or more times except Temp just like above
• \\\\t - capture this literally
• (.*? ALL-..) - Capturing any character as less as possible followed by a space followed by literal ALL- followed by any two characters

Hope this clarifies the regex. Let me know in case you have any further queries.

Demo

Sample Python Codes,

import re

s = '{"show permission allowed to 16": "show permission to 16\\nSchool permissions from group 17:student to group 16:teacher:\\n\\tAllow ALL-00\\nSchool permissions from group 18:library to group 16(Temp):teacher:\\n\\tNo Allow ALL-00\\nSchool permissions from group 20:Gym to group 16:teacher:\\n\\tCheck ALL-00\\nRTYAHY: FALSE\\nRTYAHY: FALSE\\n\\n#"}'

arr = re.findall(r'from group (\d+)(?:(?!Temp).)*?(Temp)?(?:(?!Temp).)*?\\t(.*? ALL-..)',s)
print(arr)

Prints,

[('17', '', 'Allow ALL-00'), ('18', 'Temp', 'No Allow ALL-00'), ('20', '', 'Check ALL-00')]

Edit: For listing only tuples that does not contain Temp

You will need to use this regex to avoid matching substring that contains Temp string within the match,

from group (\d+)(?:(?!Temp).)*\\t(.*? ALL-..)

Demo

Sample Python code,

import re

str1 = '{"show permission allowed to 16": "show permission to 16\\nSchool permissions from group 17:student to group 16:teacher:\\n\\tAllow ALL-00\\nSchool permissions from group 18:library to group 16(Temp):teacher:\\n\\tNo Allow ALL-00\\nSchool permissions from group 20:Gym to group 16:teacher:\\n\\tCheck ALL-00\\nRTYAHY: FALSE\\nRTYAHY: FALSE\\n\\n#"}'

arr = re.findall(r'from group (\d+)(?:(?!Temp).)*\\t(.*? ALL-..)',str1)
print(arr)

Prints,

[('17', 'Allow ALL-00'), ('20', 'Check ALL-00')]

Which does not contain the tuple having Temp