Regular Expressions 101

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/
^(?P<qualifier>[+?~-])?(?P<is_a>a)(?:[:])?(?P<a_mechanism>.+)?
/
gim
^ asserts position at start of a line
Named Capture Group qualifier
(?P<qualifier>[+?~-])?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[+?~-]
+?~-
matches a single character in the list +?~- (case insensitive)
Named Capture Group is_a
(?P<is_a>a)
a matches the character a with index 9710 (6116 or 1418) literally (case insensitive)
Non-capturing group
(?:[:])?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[:]
: matches the character : with index 5810 (3A16 or 728) literally (case insensitive)
Named Capture Group a_mechanism
(?P<a_mechanism>.+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
.
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)
Global pattern flags
g modifier: global. All matches (don't return after first match)
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
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

/
/
gim

Test String