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 a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
\d*
matches a digit (equal to [0-9])
* Quantifier — Matches 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 a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
\d*
matches a digit (equal to [0-9])
* Quantifier — Matches 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 a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
\d*
matches a digit (equal 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-]*))*))?
? Quantifier — Matches 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-]+)*))?
? Quantifier — Matches 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