Regular Expressions 101

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^[a-zA-Z]\w{5,11}$
/
^ 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)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
{5,11} matches the previous token between 5 and 11 times, as many times as possible, giving back as needed (greedy)
$ asserts position at the end of the string
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/

Test String