cs 402 lecture 3

Regular Expression (RE)
a* and a+ are the regular expressions of the Language L1 and L2
Recursive Regular Expressions :

Step 1 : Every letter of sigma E including null string is a regular expression.
Step 2 : If r1 and r2 are regular expressions then
i (r1)
ii r1 r2
iii r1+r2
iv r1* 
are also regular expressions.
Step 3: Nothing else is a regular expression.

Express any language in short way  as regular expressions.

One language has more than one regular expressions but one regular expression can make only one language.