Regular Expressions 101

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/
(?<email>(?<username>[\w\W]+)@(?<domain>[\w\W]+\.[\w\W]+|localhost))
/
gi
Named Capture Group email
(?<email>(?<username>[\w\W]+)@(?<domain>[\w\W]+\.[\w\W]+|localhost))
Named Capture Group username
(?<username>[\w\W]+)
Match a single character present in the list below
[\w\W]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\w matches any word character (equivalent to [a-zA-Z0-9_])
\W matches any non-word character (equivalent to [^a-zA-Z0-9_])
@ matches the character @ with index 6410 (4016 or 1008) literally (case insensitive)
Named Capture Group domain
(?<domain>[\w\W]+\.[\w\W]+|localhost)
1st Alternative
[\w\W]+\.[\w\W]+
Match a single character present in the list below
[\w\W]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\w matches any word character (equivalent to [a-zA-Z0-9_])
\W matches any non-word character (equivalent to [^a-zA-Z0-9_])
\. matches the character . with index 4610 (2E16 or 568) literally (case insensitive)
Match a single character present in the list below
[\w\W]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\w matches any word character (equivalent to [a-zA-Z0-9_])
\W matches any non-word character (equivalent to [^a-zA-Z0-9_])
2nd Alternative
localhost
localhost
matches the characters localhost literally (case insensitive)
Global pattern flags
g modifier: global. All matches (don't return after first match)
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gi

Test String