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I came across few Compiler Design Doubts, Please provide your cent.

$Q_1 :$ Can lexical analyser detect some/any type of errors?

I think yes, because while scanning to identify tokens, it may be the cases that a string pattern doesn't match with any keyword (or) entry in symbol table and gives error. But a detailed explaination will be appreciated.

$Q_2:$ In which phase of compilation Scope Checking happens? I guess sematic analyser, but I have no valid explaination

$Q_3: \color{navy}{A \rightarrow Aa}$ (Left linear) $\color{navy}{A \rightarrow aA}$(Right Linear). What can be said about $\color{navy}{A \rightarrow aAb}$
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Can lexical analyser detect some type of errors?
Yes it is True. Lexical Analyser detect lex error.

Can lexical analyser detect any type of errors?
No, it cant detect syntax or semantic error.

In which phase of compilation Scope Checking happens?
Semantic Analyser. Generally type checking takes place when output is coming out. It is possible onl if there is no lex and no syntax error. (http://www.cs.virginia.edu/kim/courses/cs471/lec/cs471-11-semantic.pdf)

A→Aa (Left linear),  A→aA (Right Linear).
 A→aAb it is CFG

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