Regular Expressions 101

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/
^((?:(\d{4}-\d{2}-\d{2})T(\d{2}:\d{2}:\d{2}(?:\.\d+)?))(Z|[\+-]\d{2}:\d{2})?)$
/
gm
^ asserts position at start of a line
1st Capturing Group
((?:(\d{4}-\d{2}-\d{2})T(\d{2}:\d{2}:\d{2}(?:\.\d+)?))(Z|[\+-]\d{2}:\d{2})?)
Non-capturing group
(?:(\d{4}-\d{2}-\d{2})T(\d{2}:\d{2}:\d{2}(?:\.\d+)?))
2nd Capturing Group
(\d{4}-\d{2}-\d{2})
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
T matches the character T with index 8410 (5416 or 1248) literally (case sensitive)
3rd Capturing Group
(\d{2}:\d{2}:\d{2}(?:\.\d+)?)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
Non-capturing group
(?:\.\d+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
4th Capturing Group
(Z|[\+-]\d{2}:\d{2})?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1st Alternative
Z
Z matches the character Z with index 9010 (5A16 or 1328) literally (case sensitive)
2nd Alternative
[\+-]\d{2}:\d{2}
Match a single character present in the list below
[\+-]
\+ matches the character + with index 4310 (2B16 or 538) literally (case sensitive)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
$ 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