Regular Expressions 101

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/
(?:\G(?!^);|Set-Cookie:)\s\K([^=\n])(?:=([^;\n]))?
/
gm
Non-capturing group
(?:\G(?!^);|Set-Cookie:)
1st Alternative
\G(?!^);
\G asserts position at the end of the previous match or the start of the string for the first match
Negative Lookahead
(?!^)
Assert that the Regex below does not match
^ asserts position at start of a line
; matches the character ; literally (case sensitive)
2nd Alternative
Set-Cookie:
Set-Cookie: matches the characters Set-Cookie: 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)
\K resets the starting point of the reported match. Any previously consumed characters are no longer included in the final match
1st Capturing Group
([^=\n])
Match a single character not present in the list below
[^=\n]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
= matches the character = literally (case sensitive)
\n matches a line-feed (newline) character (ASCII 10)
Non-capturing group
(?:=([^;\n]))?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
= matches the character = literally (case sensitive)
2nd Capturing Group
([^;\n])
Match a single character not present in the list below
[^;\n]
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.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String