Conditional regex

Having following regex:

Code:

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(?(?<=\AA)a|e)/g

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and test string:

Code:

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Aa
Ae
Be

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it matches `a`(line #1), `e`(line #2), `e`(line #3).
I think it should match just `a`(line #1) because there are 3 lines but only ONE string - `\AA` matches `A` only once (on the first line of the string) and therefore applies `a` pattern which matches only `a` in the fist line.