Regular Expressions 101

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`
(?P<namespace>.+/)?(?P<image>[^:]+)(?P<tag>:.+)?
`
gm
Named Capture Group namespace
(?P<namespace>.+/)?
? 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)
/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
Named Capture Group image
(?P<image>[^:]+)
Match a single character not present in the list below
[^:]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
Named Capture Group tag
(?P<tag>:.+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
: matches the character : with index 5810 (3A16 or 728) 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)
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