Regular Expressions 101

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/
\w+@(?!gmail|hotmail|yahoo)\w+(?:-\w+)?\.[A-Za-z]{2,4}
/
g
\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)
@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
Negative Lookahead
(?!gmail|hotmail|yahoo)
Assert that the Regex below does not match
1st Alternative
gmail
gmail
matches the characters gmail literally (case sensitive)
2nd Alternative
hotmail
hotmail
matches the characters hotmail literally (case sensitive)
3rd Alternative
yahoo
yahoo
matches the characters yahoo 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)
Non-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 4510 (2D16 or 558) 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)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Match a single character present in the list below
[A-Za-z]
{2,4} matches the previous token between 2 and 4 times, as many times as possible, giving back as needed (greedy)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
g

Test String