Convert string to date & select rows with certain feature

Hi guys, just can't figure this out:

1. Suppose I have 100 rows of data, rows are such as:
"ABMAR99", "ABMAR02", "ABDEC03", etc.

How to convert for example, "ABMAR99" to the first wednesday
in March 1999, which is, "1999-03-03"?

2. Suppose other than rows like "ABMAR99", "ABMAR02", "ABDEC03", I also
have rows like "AB_MLTE", "ABMAR2C0",etc, and I only want to keep the
rows
in the format ("ABMAR99", "ABMAR02", "ABDEC03").

In other words:

(1) I only want to keep rows with the exact length of 7.
(2) For rows with the exact length of 7, I want to delete rows whose
third letter is "_".

Thanks a lot, appreciate your help!