Regular Expressions 101

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/
^(?P<major>0|[1-9]\d)\.(?P<minor>0|[1-9]\d)\.(?P<patch>0|[1-9]\d)(?:-(?P<prerelease>(?:0|[1-9]\d|\d[a-zA-Z-][0-9a-zA-Z-])(?:\.(?:0|[1-9]\d|\d[a-zA-Z-][0-9a-zA-Z-]))*))?(?:\+(?P<buildmetadata>[0-9a-zA-Z-](?:\.[0-9a-zA-Z-])*))?$
/
gm
^ asserts position at start of a line
Named Capture Group major
(?P<major>0|[1-9]\d)
1st Alternative
0
0 matches the character 0 literally (case sensitive)
2nd Alternative
[1-9]\d
Match a single character present in the list below
[1-9]
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
\d
matches a digit (equivalent to [0-9])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\. matches the character . literally (case sensitive)
Named Capture Group minor
(?P<minor>0|[1-9]\d)
1st Alternative
0
0 matches the character 0 literally (case sensitive)
2nd Alternative
[1-9]\d
Match a single character present in the list below
[1-9]
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
\d
matches a digit (equivalent to [0-9])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\. matches the character . literally (case sensitive)
Named Capture Group patch
(?P<patch>0|[1-9]\d)
1st Alternative
0
0 matches the character 0 literally (case sensitive)
2nd Alternative
[1-9]\d
Match a single character present in the list below
[1-9]
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
\d
matches a digit (equivalent to [0-9])
Non-capturing group
(?:-(?P<prerelease>(?:0|[1-9]\d|\d[a-zA-Z-][0-9a-zA-Z-])(?:\.(?:0|[1-9]\d|\d[a-zA-Z-][0-9a-zA-Z-]))*))?
? 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)
Named Capture Group prerelease
(?P<prerelease>(?:0|[1-9]\d|\d[a-zA-Z-][0-9a-zA-Z-])(?:\.(?:0|[1-9]\d|\d[a-zA-Z-][0-9a-zA-Z-]))*)
Non-capturing group
(?:\+(?P<buildmetadata>[0-9a-zA-Z-](?:\.[0-9a-zA-Z-])*))?
? 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)
Named Capture Group buildmetadata
(?P<buildmetadata>[0-9a-zA-Z-](?:\.[0-9a-zA-Z-])*)
$ asserts position at the end of a line
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