C:\>perl -le "$_ =~ /{}/;"

C:\>perl -le "$_ =~ /.+{}/;"

C:\>perl -le "$_ =~ /a+{}/;"

C:\>perl -le "$_ =~ /(a+)?{}/;"

C:\>perl -le "$_ =~ /a{}/;"
Unescaped left brace in regex is deprecated, passed through in regex; marked by
<-- HERE in m/a{ <-- HERE }/ at -e line 1.

C:\>perl -le "$_ =~ /\d{}/;"
Unescaped left brace in regex is deprecated, passed through in regex; marked by
<-- HERE in m/\d{ <-- HERE }/ at -e line 1.

C:\>