Substitutions and capture groups are very useful in implementing simple chatbots like ELIZA (Weizenbaum, 1966). Recall that ELIZA simulates a Rogerian psychologist by carrying on conversations like the following: User 1 : Men are all alike. ELIZA 1 : IN WHAT WAY User 2 : They're always bugging us about something or other. ELIZA 2 : CAN YOU THINK OF A SPECIFIC EXAMPLE User 3 : Well, my boyfriend made me come here. ELIZA 3 : YOUR BOYFRIEND MADE YOU COME HERE User 4 : He says I'm depressed much of the time. ELIZA 4 : I AM SORRY TO HEAR YOU ARE DEPRESSED