Regular Expressions 101

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/
^[a-zA-Z][a-zA-Z0-9-_\.]{1,20}$
/
^ asserts position at start of the string
Match a single character present in the list below
[a-zA-Z]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
Match a single character present in the list below
[a-zA-Z0-9-_\.]
{1,20} matches the previous token between 1 and 20 times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
_ matches the character _ with index 9510 (5F16 or 1378) literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/

Test String