/(fred|wilma) (flintstone) \2/ 
# match "fred" or "wilma" and put them in bucket 1
# followed by "flintstone" and put that in bucket 2
# then match whatever is in bucket 2. Which in this 
# case must be "flintstone" so the regex is 
# equivelent to
# /(fred|wilma) (flintstone) flintsone/

/(fred|wilma) (flintstone) \1/ 
# match "fred" or "wilma" and put them in bucket 1
# followed by "flintstone" and put that in bucket 2
# then match whatever is in bucket 1. Which in this 
# case could be "fred" or "wilma" so the regex is 
# equivelent to one of the following:
# /(fred) (flintstone) fred/
# /(wilma) (flintstone) wilma/

##</code><code>##

#!perl -l
print $_="the thing that that thing does";
/(\w+) that/    and printf '$&=%-20s $1=%-10s %s',$&,$1,$/;
/($1) (\w+)/    and printf '$&=%-20s $1=%-10s $2=%-10s %s',$&,$1,$2,$/;
/$1 (\w+) (\1)/ and printf '$&=%-20s $1=%-10s $2=%-10s %s',$&,$1,$2,$/;
__END__
the thing that that thing does
$&=thing that           $1=thing      
$&=thing that           $1=thing      $2=that       
$&=thing that that      $1=that       $2=that