LEXICAL PHASE
1. Develop a regular expression (RE) that detects the longest string over the alphabet {a-z}
with the following properties:
i). The string begins with an ‘a’ character and ends with a ‘z’ character;
ii). After the first ‘a’, the string can include an alternation of two sub sequences, namely
a sequence that begins with a ‘b’ character followed by zero or more ‘s’ characters ending with
an ‘e’ character and a subsequence that begins with a ‘c’ character but does not have any ‘s’
character until a terminating ‘t’ character.
iii). If at any point in the subsequence there is a ‘x’ character, that specific sequence is
considered terminated, i.e., the ‘x’ acts as the ‘e’ character in the first type of sequence and as
the ‘t’ character in the second type of sequence.
iv). The two subsequences cannot be nested but can be repeated in any alternating order.
As an example, the string “absssecaaabbbafghtbsez” is to be accepted as well as the string
“abssxcfftz”
2. Write the Regular Expression for all the strings over {a,b} such that the string contains equal
occurrence of the substrings ab and ba.
3. a) Suppose you have a machine M, a program P written in language L, and a compiler for L
that runs on M. Draw a tombstone diagram that shows how to run P on M.
b) Suppose you have a machine M, a program P written in language L1, an interpreter for L1
written in language L2, and an interpreter for L2 that runs on M. Draw a tombstone diagram
that shows how to run P on M.
c) Suppose you have two machines M1 and M2, a program P written in language L, an
interpreter for L that runs on M1, and a translator from M1 to M2 written in L. Draw a
tombstone diagram that shows how to run P on M2.
