Regular Expressions 101

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/
@e(?:[^u]reka|u[^r]eka|ur[^e]ka|ure[^k]a|urek[^a]|eurk[^a])\.com
/
gm
@e
matches the characters @e literally (case sensitive)
Non-capturing group
(?:[^u]reka|u[^r]eka|ur[^e]ka|ure[^k]a|urek[^a]|eurk[^a])
1st Alternative
[^u]reka
Match a single character not present in the list below
[^u]
u matches the character u with index 11710 (7516 or 1658) literally (case sensitive)
reka
matches the characters reka literally (case sensitive)
2nd Alternative
u[^r]eka
u matches the character u with index 11710 (7516 or 1658) literally (case sensitive)
Match a single character not present in the list below
[^r]
r matches the character r with index 11410 (7216 or 1628) literally (case sensitive)
eka
matches the characters eka literally (case sensitive)
3rd Alternative
ur[^e]ka
ur
matches the characters ur literally (case sensitive)
Match a single character not present in the list below
[^e]
e matches the character e with index 10110 (6516 or 1458) literally (case sensitive)
ka
matches the characters ka literally (case sensitive)
4th Alternative
ure[^k]a
ure
matches the characters ure literally (case sensitive)
Match a single character not present in the list below
[^k]
k matches the character k with index 10710 (6B16 or 1538) literally (case sensitive)
a matches the character a with index 9710 (6116 or 1418) literally (case sensitive)
5th Alternative
urek[^a]
urek
matches the characters urek literally (case sensitive)
Match a single character not present in the list below
[^a]
a matches the character a with index 9710 (6116 or 1418) literally (case sensitive)
6th Alternative
eurk[^a]
eurk
matches the characters eurk literally (case sensitive)
Match a single character not present in the list below
[^a]
a matches the character a with index 9710 (6116 or 1418) literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
com
matches the characters com literally (case sensitive)
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