The below regex will match the strings that start and end with an alpha character.
/^[a-z].*[a-z]$/igm
The a string also starts and ends with an alpha character, right? Then you have to use the below regex.
/^[a-z](.*[a-z])?$/igm
DEMO
Explanation:
^ # Represents beginning of a line
[a-z] # Alphabetic character
.* # Any character 0 or more times
[a-z] # Alphabetic character
$ # End of a line
i # Case-insensitive match
g # Global
m # Multiline