'1234'
/(?<P>(?&V))(?<Q>.)(?(DEFINE)(?<V>...))/

&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    (?<P>           &#8592;[0m  &#8592;[36;40m    The start of a named capturing block (also $1)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m      (?&V)         &#8592;[0m  &#8592;[36;40m      Match a call to the subpattern named <V>&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    )               &#8592;[0m  &#8592;[36;40m    The end of the named capturing block&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    (?<Q>           &#8592;[0m  &#8592;[36;40m    The start of a named capturing block (also $2)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m      .             &#8592;[0m  &#8592;[36;40m      Match any character (except newline)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    )               &#8592;[0m  &#8592;[36;40m    The end of the named capturing block&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    (?(DEFINE)      &#8592;[0m  &#8592;[36;40m    The start of a definition block (skipped during matching)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m      (?<V>         &#8592;[0m  &#8592;[36;40m      The start of a named capturing block (also $3)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m        .           &#8592;[0m  &#8592;[36;40m        Match any character (except newline)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m        .           &#8592;[0m  &#8592;[36;40m        Match any character (except newline)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m        .           &#8592;[0m  &#8592;[36;40m        Match any character (except newline)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m      )             &#8592;[0m  &#8592;[36;40m      The end of the named capturing block&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    )               &#8592;[0m  &#8592;[36;40m    The end of definition block&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m