Regular Expressions 101

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/
^\{((?:(?:(?!\}).)+)(?:(?:\}\})?(?:(?:(?!\}).)+))+)\}(?!\})
/
gm
^ asserts position at start of a line
\{ matches the character { with index 12310 (7B16 or 1738) literally (case sensitive)
1st Capturing Group
((?:(?:(?!\}).)+)(?:(?:\}\})?(?:(?:(?!\}).)+))+)
Non-capturing group
(?:(?:(?!\}).)+)
Non-capturing group
(?:(?!\}).)+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Negative Lookahead
(?!\})
Assert that the Regex below does not match
\} matches the character } with index 12510 (7D16 or 1758) literally (case sensitive)
. matches any character (except for line terminators)
Non-capturing group
(?:(?:\}\})?(?:(?:(?!\}).)+))+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:\}\})?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\} matches the character } with index 12510 (7D16 or 1758) literally (case sensitive)
\} matches the character } with index 12510 (7D16 or 1758) literally (case sensitive)
Non-capturing group
(?:(?:(?!\}).)+)
Non-capturing group
(?:(?!\}).)+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Negative Lookahead
(?!\})
Assert that the Regex below does not match
\} matches the character } with index 12510 (7D16 or 1758) literally (case sensitive)
. matches any character (except for line terminators)
\} matches the character } with index 12510 (7D16 or 1758) literally (case sensitive)
Negative Lookahead
(?!\})
Assert that the Regex below does not match
\} matches the character } with index 12510 (7D16 or 1758) 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

/
/
gm

Test String