dict: Speed up word completion with common use cases and large word lists.

/usr/share/dict/words has 479829 entries on my box which is too much
for practical compgen -W usage, narrow it down with grep in common use
cases.

completions/dict

@@ -54,8 +54,17 @@ _dict()
     esac
 
     local dictfile=/usr/share/dict/words
-    [[ -r $dictfile ]] && \
-        COMPREPLY=( $( compgen -W '$( cat $dictfile )' -- "$cur" ) )
+    if [[ -r $dictfile ]]; then
+        # Dictfile may be too large for practical compgen -W usage, so narrow
+        # it down with grep if $cur looks like something that's safe to embed
+        # in a pattern instead.
+        if [[ $cur == +([-A-Za-z0-9/.]) ]]; then
+            COMPREPLY=( $( compgen -W \
+                '$( command grep "^${cur//./\\.}" $dictfile )' -- "$cur" ) )
+        else
+            COMPREPLY=( $( compgen -W '$( cat $dictfile )' -- "$cur" ) )
+        fi
+    fi
 } &&
 complete -F _dict -o default dict rdict