regex - How to match only odd occurrences of a character at the end of the line using grep -

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for example, i'm matching odd occurrences of 'a'. "helloaaa" should match while "helloaaaa" should not match.

i've tried "(aa)*a$" , without -e option on bash.

your problem helloaaaa matches because of last 3 as:

helloaaaa       ===

to avoid need make sure previous character not a:

grep -e '[^a](aa)*a$' filename

here i'm assuming line isn't entirely as. if entire line can as can use regular expression instead:

grep -e '(^|[^a])(aa)*a$' filename

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