From "perldoc perlre"

           \G  Match only at pos() (e.g. at the end-of-match position
               of prior m//g)

[snip]

       The "\G" assertion can be used to chain global matches (using "
+m//g"),
       as described in "Regexp Quote-Like Operators" in perlop.  It is
+ also
       useful when writing "lex"-like scanners, when you have several 
+patterns
       that you want to match against consequent substrings of your st
+ring,
       see the previous reference.  The actual location where "\G" wil
+l match
       can also be influenced by using "pos()" as an lvalue: see "pos"
+ in
       perlfunc. Currently "\G" is only fully supported when anchored 
+to the
       start of the pattern; while it is permitted to use it elsewhere
+, as in
       "/(?<=\G..)./g", some such uses ("/.\G/g", for example) current
+ly cause
       problems, and it is recommended that you avoid such usage for n
+ow.
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