Regular Expressions 101

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"
^[a-z0-9]{1,5}@[a-z0-9]{1,5}.[a-z]{1,3}$
"
^ asserts position at start of the string
Match a single character present in the list below
[a-z0-9]
{1,5} matches the previous token between 1 and 5 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)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
Match a single character present in the list below
[a-z0-9]
{1,5} matches the previous token between 1 and 5 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)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
. matches any character (except for line terminators)
Match a single character present in the list below
[a-z]
{1,3} matches the previous token between 1 and 3 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)
$ 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.

Regular Expression
No Match

r"
"

Test String