import re


def find_letter_index(string):
    pattern = r'([a-zA-Z])x\b'
    matches = re.findall(pattern, string)

    letter_indices = []
    for match in matches:
        letter_index = string.index(match)
        letter_indices.append(letter_index)

    return letter_indices


# 示例使用
string = "ax+b + cx - dx + e^x"
indices = find_letter_index(string)
print(indices)