Regular Expressions 101

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/
^((?!\.)[\w-_.]*[^.])(@\w+)(\.\w+(\.\w+)?[^.\W])$
/
gm
^ asserts position at start of a line
1st Capturing Group
((?!\.)[\w-_.]*[^.])
Negative Lookahead
(?!\.)
Assert that the Regex below does not match
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Match a single character present in the list below
[\w-_.]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\w matches any word character (equivalent to [a-zA-Z0-9_])
- 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 a single character in the list _. (case sensitive)
Match a single character not present in the list below
[^.]
. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
2nd Capturing Group
(@\w+)
@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
3rd Capturing Group
(\.\w+(\.\w+)?[^.\W])
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
4th Capturing Group
(\.\w+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character not present in the list below
[^.\W]
. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\W matches any non-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