/**
 * file: main.c
 * author: 简单教程(www.twle.cn)
 */

#include<stdio.h>

int main()
{
    char line[150];
    int i, j;
    printf("输入一段字符串: ");
    fgets(line, (sizeof line / sizeof line[0]), stdin);

    for(i = 0; line[i] != '\0'; ++i)
    {
        while (!( (line[i] >= 'a' && line[i] <= 'z') || (line[i] >= 'A' && line[i] <= 'Z') || line[i] == '\0') )
        {
            for(j = i; line[j] != '\0'; ++j)
            {
                line[j] = line[j+1];
            }
            line[j] = '\0';
        }
    }
    printf("只留字母后的结果如下: ");
    puts(line);
    return 0;
}

$ gcc main.c && ./a.out
输入一段字符串: 2worjsa2oeqj  oelf, o2jsljfo2jllf
只留字母后的结果如下: worjsaoeqjoelfojsljfojllf