Regular Expressions 101

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/
(?:FN:Tom )((.|\n)*)(?:\nEND:VCARD)
/
gm
Non-capturing group
(?:FN:Tom )
FN:Tom
matches the characters FN:Tom literally (case sensitive)
1st Capturing Group
((.|\n)*)
2nd Capturing Group
(.|\n)*
* 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
1st Alternative
.
. matches any character (except for line terminators)
2nd Alternative
\n
\n matches a line-feed (newline) character (ASCII 10)
Non-capturing group
(?:\nEND:VCARD)
\n matches a line-feed (newline) character (ASCII 10)
END:VCARD
matches the characters END:VCARD 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