Regular Expressions 101

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/
^((?![\d_])([\w]){8,30})$
/
gm
^ asserts position at start of a line
1st Capturing Group
((?![\d_])([\w]){8,30})
Negative Lookahead
(?![\d_])
Assert that the Regex below does not match
Match a single character present in the list below
[\d_]
\d matches a digit (equivalent to [0-9])
_ matches the character _ with index 9510 (5F16 or 1378) literally (case sensitive)
2nd Capturing Group
([\w]){8,30}
{8,30} matches the previous token between 8 and 30 times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
Match a single character present in the list below
[\w]
\w matches any word character (equivalent to [a-zA-Z0-9_])
$ 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