Regular Expressions 101

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/
^(?:0|[1-8]\d?|90?)(?:\.\d+)?$
/
gm
^ asserts position at start of a line
Non-capturing group
(?:0|[1-8]\d?|90?)
1st Alternative
0
0 matches the character 0 with index 4810 (3016 or 608) literally (case sensitive)
2nd Alternative
[1-8]\d?
Match a single character present in the list below
[1-8]
1-8 matches a single character in the range between 1 (index 49) and 8 (index 56) (case sensitive)
\d
matches a digit (equivalent to [0-9])
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
3rd Alternative
90?
9 matches the character 9 with index 5710 (3916 or 718) literally (case sensitive)
0
matches the character 0 with index 4810 (3016 or 608) literally (case sensitive)
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)
$ 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.

Regular Expression
No Match

/
/
gm

Test String