151 questions linked to/from How to prove that a language is not regular?
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### Can't tell whether the following language is regular or not: [duplicate]

I have to decide if the following language is regular or not. I suspect it is not regular, so I try using pumping lemma to prove it, but something goes wrong. Any help on how to use pumping lemma on ...
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### Pumping lemma for non regular languages [duplicate]

I read that pumping lemma is sufficient condition to prove non regularity of languages but not necessary condition. I know the first part that it is sufficient is true but not able to understand why ...
### proving L = {$a^{100}yy^r : \forall y \in$ {a,b}*} is not regular
I need to prove that L = {$a^{100}yy^r : \forall y \in$ {a,b}*} is not regular. i have tried using pumping lemma but couldn't get far with it. Any help in where i should go with it?