given that,
<pairlet> :: = <letter><letter> | <pairlet><letter><letter>
∴ <pairlet> = (<letter><letter>)$^+$ ------> (1)
Generates all strings which are even length and all are letters and length grater than 0.
<pairdig> :: = <digit><digit> | <pairdig><digit><digit>
∴ <pairdig> = (<digit><digit>)$^+$ ------> (2)
Generates all strings which are even length and all are digits and length grater than 0.
<word> ::= <letter> | <letter><pairletter> | <letter><pairdigit>
<word> ::= <letter> | <letter> (<letter><letter>)$^+$ | <letter> (<digit><digit>)$^+$
generates, following types of strings
i) all strings which are length odd and those are letters only.
ii) all the strings which are length odd and those are starts with one letter and remaining are digits only.