$ perl -we 'q{xxx.xxx} =~ /\b(xxx\.xxx)\b/; print "|$1|\n"'
|xxx.xxx|
$ perl -we 'q{.xxx} =~ /\b(\.xxx)\b/; print "|$1|\n"'
Use of uninitialized value $1 in concatenation (.) or string at -e line 1.
||