Regular Expressions 101

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/
^----[- ](?:BEGIN|END) (?:([A-Z][A-Z0-9]+) )?(PRIVATE|PUBLIC) KEY[ -]----$
/
mg
^ asserts position at start of a line
----
matches the characters ---- literally (case sensitive)
Match a single character present in the list below
[- ]
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
matches the character with index 3210 (2016 or 408) literally (case sensitive)
Non-capturing group
(?:BEGIN|END)
1st Alternative
BEGIN
BEGIN
matches the characters BEGIN literally (case sensitive)
2nd Alternative
END
END
matches the characters END literally (case sensitive)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
Non-capturing group
(?:([A-Z][A-Z0-9]+) )?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1st Capturing Group
([A-Z][A-Z0-9]+)
Match a single character present in the list below
[A-Z]
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
Match a single character present in the list below
[A-Z0-9]
+ 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 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
2nd Capturing Group
(PRIVATE|PUBLIC)
1st Alternative
PRIVATE
PRIVATE
matches the characters PRIVATE literally (case sensitive)
2nd Alternative
PUBLIC
PUBLIC
matches the characters PUBLIC literally (case sensitive)
KEY
matches the characters KEY literally (case sensitive)
Match a single character present in the list below
[ -]
matches the character with index 3210 (2016 or 408) literally (case sensitive)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
----
matches the characters ---- literally (case sensitive)
$ asserts position at the end of a line
Global pattern flags
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/
mg

Test String