A complete example of <?{}>.

See issue 1757

Author: fluca1978

doc/Language/regexes.pod6

@@ -1324,6 +1324,9 @@ a semantic evaluation of the match before the regular expression continues. In o
 to check in aboolean context a part of a regular expression and therefore invalidate the whole match (or allow it
 to continue) even if the match succeed from a syntactic point of view.
 
+In particular the C«<?{}>» operator requires a C<True> value in order to allow the regular expression to match, while its
+negate form  C«<!{}>» requires a C<False> value.
+
 In order to demonstrate the above operator, please consider the following example that involves
 a simple IPv4 address matching:
 
@@ -1347,6 +1350,16 @@ $localhost ~~ / ^ <ipv4-octet> ** 4 % "." $ /;
 say $/<ipv4-octet>;   # OUTPUT: Nil
 =end code
 
+From the above examples, it should be clear that it is possible to improve the semantic check, for instance
+ensuring that each I<octet> is really a valid IPv4 octect:
+
+=begin code
+my $localhost = '127.0.0.1';
+my regex ipv4-octet { \d ** 1..3 <?{ $/.Int <= 255 && $/.Int >= 0 }> }
+$localhost ~~ / ^ <ipv4-octet> ** 4 % "." $ /;
+say $/<ipv4-octet>;   # OUTPUT: [｢127｣ ｢0｣ ｢0｣ ｢1｣]
+=end code
+
 
 =head1 Adverbs