You could use regex capturing group.
if(Regex.IsMatch(temp,@"^([a-zA-Z])[a-zA-Z]{3,}\1$"))
It should match the words which starts and endswith the same letter and the word must contain atleast 5 letters. For greater than 5 letters, just change the number 3 to 4.
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solved Check words that start and end with same letter in C#