Regular Expressions 101

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/
^(90|[0-8][0-9]?)$
/
gm
^ asserts position at start of a line
1st Capturing Group
(90|[0-8][0-9]?)
1st Alternative
90
90
matches the characters 90 literally (case sensitive)
2nd Alternative
[0-8][0-9]?
Match a single character present in the list below
[0-8]
0-8 matches a single character in the range between 0 (index 48) and 8 (index 56) (case sensitive)
Match a single character present in the list below
[0-9]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
$ 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

Code Generator

Generated Code

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Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report.
For a full regex reference for Java, please visit: https://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html