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If language
L = ɸ = { } then r (regular expression) = ɸ
L = {λ} the r = λ
L = {a} then r = a
L = {a, b} then r = a + b
L = {a, b, aa, bb} then r = a + b + aa + bb
L = {λ, a, b} then r = λ + a + b
If the language is finite then we write a regular expression for it. Language finite means it has finite no. of strings.
One language has more than one Regular Expression.
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But one Regular Expression has a unique Regular Language.
Now one language has many machines.
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Same as a Regular Expression has many machines.
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Example:
L = ɸ ∑ = {a, b} RE = ɸ
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So, one Regular Expression (RE) may have several machines (DFA or NFA) or one machine may several RE.
Example:
RE = (a + b)*
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So, some RE may more than one machine.
Example:
For one machine may more than one Regular Expression.
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RE1 = (a + ba*b)*ba* and RE2 = a*b(a + ba*b)*
We can write another regular expression for this machine RE3 = a*b(a + ba*b)*a*
So, one machine may one or more regular expressions.
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But one Regular Expression has one unique language.
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Because we know L = {a, b} RE = a + b and L = {a, ab, ba} RE = a + ab + ba
Because the language is finite and we can write a RE for it then it is a regular language.
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String length wise ordering is called proper ordering. Another way we can order is alphabetically ordering. = {λ, a, aa, aaa, ab,….. , bbb}