compiler design

2.1 Write regular expressions for the following character sets, or give reasons why no regular expression can be written: 2. All strings of lowercase letters that begin and end in a. D. All strings of lowercase letters that either begin or end in a (or both). C. All strings of digits that contain no leading zeros. d. All strings of digits that represent even numbers. e. All strings of digits such that all the 2's occur before all the 9's. f. All strings of a's and b's that contain no three consecutive b's. 8. All strings of a's and b's that contain an odd number of a's or an odd number of b's (or both). h. All strings of a's and b's that contain an even number of a's and an even number of b's. I. All strings of a's and b's that contain exactly as many a's as b's. Write English descriptions for the languages generated by the following regular expres- sions: a. (alb)*a(able) b. (A|B... 1z) (alb)... |z) * c. (aa/b)(abb) d. (0111... 19/ABC|PE|P)+(x|x) 22 W