Regex validate correct ISO8601 date string with time
Based on an earlier answer of mine, you could do this and be pretty darn strict:
^(?:[1-9]\d{3}-(?:(?:0[1-9]|1[0-2])-(?:0[1-9]|1\d|2[0-8])|(?:0[13-9]|1[0-2])-(?:29|30)|(?:0[13578]|1[02])-31)|(?:[1-9]\d(?:0[48]|[2468][048]|[13579][26])|(?:[2468][048]|[13579][26])00)-02-29)T(?:[01]\d|2[0-3]):[0-5]\d:[0-5]\d(?:Z|[+-][01]\d:[0-5]\d)$
Debuggex Demo
Slightly monstrous but it checks for valid dates including leap-year (Proleptic Gregorian), works for years 1000-9999, checks for invalid times like 25:30 or 21:94 and a maximum UTC offset of +/-19:59 (or a Z).
(right now more than +14:00 or -12:00 doesn't happen, but it might in the future).
For completion: This answer only supports a subset of the ISO8601 standard based on the examples OP gave. Which is the extended notation with seconds in the time section and minutes in the UTC offset. For brevity it does not support basic notation where dashes and colons are omitted, or the omitting of minutes in the UTC offset or the smallest unit. Nor is there support for ordinal dates (day-of-year) or year-week-dayofweek notations.
An extended version of the regex that supports basic notation, ordinal and the omitting of seconds / UTC-offset minutes lives here.
Based on the previous answer, this regex handles the fraction of seconds.
^(?:[1-9]\d{3}-(?:(?:0[1-9]|1[0-2])-(?:0[1-9]|1\d|2[0-8])|(?:0[13-9]|1[0-2])-(?:29|30)|(?:0[13578]|1[02])-31)|(?:[1-9]\d(?:0[48]|[2468][048]|[13579][26])|(?:[2468][048]|[13579][26])00)-02-29)T(?:[01]\d|2[0-3]):[0-5]\d:[0-5]\d(?:\.\d{1,9})?(?:Z|[+-][01]\d:[0-5]\d)$
Debuggex Demo
In this link we see the different ISO 8601 formats based on different values we would like to include.
For the example in the question, 2015-01-17T18:23:02+00:00, below regex should work.
[0-9]{4}-[0-9]{2}-[0-9]{2}T([0-9]{2}:){2}[0-9]{2}[+|-][0-9]{2}:[0-9]{2}
Here [+|-] is for possible time zone offsets.
If you just want to match time zones use this:
^[+|-][0-1][0-9]:[0-5][0-9]$
The max is + or - 19:59 which is sufficient