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25. Additional 3-21: Find a regular expression to describe L, where L = The set of strings over the alphabet {a, b, c, d} where every ‘‘c’’ is immediately preceded and also followed by a ‘‘b’’ (i.e. every ‘‘c’’ has the symbol ‘‘b’’ right next to it on both sides) So, for example ... These strings are in L: o a, b, d, bcb, abbbbcbddabcb, abd, dbcbcb, bcbcb and these strings are NOT in L: o abc, abcbabc, asdc, cbbb, acb 26. Additional 3-22: Find a regular expression to describe m, where M = the set of strings over the alphabet {a,b,c,d} which start with the letter ‘a’ or the letter ‘b’ and also contain exactly one c For example, the following strings are in M: • ac, bc, abc, abcba, bcbd, bbbbbbc, aaaaaabbbbcdd, bbbbcaaa and these are NOT in M: • abcbabc, a, b, c, d, ca, dbbbacbba, bcabcd

          25. Additional 3-21:
Find a regular expression to describe L, where L = The set of strings over the alphabet {a, b, c, d} where every ‘‘c’’ is immediately preceded and also followed by a ‘‘b’’ (i.e. every ‘‘c’’ has the symbol ‘‘b’’ right next to it on both sides) So, for example ... These strings are in L:
o a, b, d, bcb, abbbbcbddabcb, abd, dbcbcb, bcbcb
and these strings are NOT in L:
o abc, abcbabc, asdc, cbbb, acb

26. Additional 3-22:
Find a regular expression to describe m, where M = the set of strings over the alphabet {a,b,c,d} which start with the letter ‘a’ or the letter ‘b’ and also contain exactly one c
For example, the following strings are in M:
• ac, bc, abc, abcba, bcbd, bbbbbbc, aaaaaabbbbcdd, bbbbcaaa
and these are NOT in M:
• abcbabc, a, b, c, d, ca, dbbbacbba, bcabcd

25. Additional 3-21:
Find a regular expression to describe L, where L = The set of strings over the alphabet a, b, c, d where every ‘‘c’’ is immediately preceded and also followed by a ‘‘b’’ (i.e. every ‘‘c’’ has the symbol ‘‘b’’ right next to it on both sides) So, for example ... These strings are in L:
o a, b, d, bcb, abbbbcbddabcb, abd, dbcbcb, bcbcb
and these strings are NOT in L:
o abc, abcbabc, asdc, cbbb, acb

26. Additional 3-22:
Find a regular expression to describe m, where M = the set of strings over the alphabet a,b,c,d which start with the letter ‘a’ or the letter ‘b’ and also contain exactly one c
For example, the following strings are in M:
• ac, bc, abc, abcba, bcbd, bbbbbbc, aaaaaabbbbcdd, bbbbcaaa
and these are NOT in M:
• abcbabc, a, b, c, d, ca, dbbbacbba, bcabcd

Added by Amanda C.

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