find and replace

[asad]

Hi guys,
I would like to tell you that I use Arabic language and it write from right to left, and I asked my question English letters and words to be easy for you

My question is:
I want to find each word beginning with "ast" provided the number of letters seven or more, and then replace the first letter of "a" to "e"
sample: Find: astfsar Replace with: estfsar

thanks in advance

[gmayor]

I don't know about Arabic, but in English you would search for

a(st[a-z]{4,}>)
with the wildcard option set and replace with
e\1