Regular Expressions 101

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/
^((?!WEBSITE1\.com|WEBSITE2\.com).)*$
/
gm
^ asserts position at start of a line
1st Capturing Group
((?!WEBSITE1\.com|WEBSITE2\.com).)*
* matches the previous token between zero and unlimited 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
Negative Lookahead
(?!WEBSITE1\.com|WEBSITE2\.com)
Assert that the Regex below does not match
1st Alternative
WEBSITE1\.com
WEBSITE1
matches the characters WEBSITE1 literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
com
matches the characters com literally (case sensitive)
2nd Alternative
WEBSITE2\.com
WEBSITE2
matches the characters WEBSITE2 literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
com
matches the characters com literally (case sensitive)
. matches any character (except for line terminators)
$ 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.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String