Regular Expressions 101

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/
\w+(?:\.+\w+)+
/
gm
\w+
matches any word character (equal to [a-zA-Z0-9_])
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:\.+\w+)+
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
\.+
matches the character . literally (case sensitive)
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
\w+
matches any word character (equal to [a-zA-Z0-9_])
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
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