Regular Expressions 101

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"
__([a-zA-Z_]+)__\s*=\s*(?:\"|')((?<=\").+(?=\")|(?<=').+(?='))
"
gm
__
matches the characters __ literally (case sensitive)
1st Capturing Group
([a-zA-Z_]+)
Match a single character present in the list below
[a-zA-Z_]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
_ matches the character _ with index 9510 (5F16 or 1378) literally (case sensitive)
__
matches the characters __ literally (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v  ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
= matches the character = with index 6110 (3D16 or 758) literally (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v  ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:\"|')
1st Alternative
\"
\" matches the character " with index 3410 (2216 or 428) literally (case sensitive)
2nd Alternative
'
' matches the character ' with index 3910 (2716 or 478) literally (case sensitive)
2nd Capturing Group
((?<=\").+(?=\")|(?<=').+(?='))
1st Alternative
(?<=\").+(?=\")
Positive Lookbehind
(?<=\")
Assert that the Regex below matches
\" matches the character " with index 3410 (2216 or 428) literally (case sensitive)
.
matches any character (except for line terminators)
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Positive Lookahead
(?=\")
Assert that the Regex below matches
\" matches the character " with index 3410 (2216 or 428) literally (case sensitive)
2nd Alternative
(?<=').+(?=')
Positive Lookbehind
(?<=')
Assert that the Regex below matches
' matches the character ' with index 3910 (2716 or 478) literally (case sensitive)
.
matches any character (except for line terminators)
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Positive Lookahead
(?=')
Assert that the Regex below matches
' matches the character ' with index 3910 (2716 or 478) literally (case sensitive)
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"
gm

Test String