匹配字符串中某个字符出现的次数

def count(string,substring):
    count = 0
    index = 0
    while index<len(string):
        if string[index:index+len(substring)] == substring:
            count+=1
            index+=len(substring)
        else:
            index+=1
    return count

print(count('znxghdfgzhang2134zhang3543qwertncm123cmc12041nmb','zhang'))