bob@here the workspace

nothing matches

Rleft.side new.stuff this is a comment 

Figure 2. How ``x'' and ``y'' become ``a''.

x the workspace

exact match, so: TRUE

Rx y  first rule rewritten
y  workspace to be this

exact match, so TRUE

Ry z  second rule

Figure 3. How ``x'' and ``y'' become ``a'', another view:

x becomes  y becomes  z becomes  a
y becomes  z becomes  a
z becomes  a

Figure 4. An operator in the left-hand side.

R$+ $@match
 
lhs rhs

Figure 5. Tokenizing an address with user and host portions:

gw@wash.dc.gov

tokenized into

"gw" "@" "wash" "." "dc" "." "gov" in the workspace

Figure 6. A good versus a bad address:

gw@wash.dc.gov  $+ matches
@wash.dc.gov  $+ also matches an incomplete address

Figure 7. How $+@$+ matches the workspace:

workspace pattern
"gw" "$+"  match one or more
"@" "@"  exact match
"wash" "$+"  matches one
"."  or more
"dc"
"."
"gov"

Figure 8. Why $+@$+ doesn't match @wash.dc.gov:

workspace pattern
"@" "$+"  match one
"wash"  or more
"."
"dc"
"."
"gov"
"@"  match exactly (fails!)
"$+"

Figure 9. The flow of an address through the rules:

user@host.domain does not match the rule:

R@ $@one

so try the next rule:

R@$+ $@two

which doesn't match so try the next rule, which does:

R$+@$+ $@three