Replace substring with another string

Write a program to replace a substring with another string in the given string.

C    PYTHON

#include<stdio.h>
#include<string.h>
int main() {
  char str[100],substr[20], replacestr[30];
  int i,j,k,l;
  fgets(str,100,stdin);
  fgets(substr,20,stdin);
  fgets(replacestr,30,stdin);
  for(i=0;i<strlen(str);i++){
    for(j=0,k=i;j<strlen(substr)-1;j++,k++){
      if(str[k]!=substr[j])
        break;
    }
//Substring is found in string
    if(j==strlen(substr)-1){   
//subtring and replace string have same length
      if(strlen(substr)==strlen(replacestr)){ 
//then replace substring with another string
        for(j=0,k=i;j<strlen(substr)-1;j++,k++)   
          str[k] = replacestr[j];
      } //length of substring is greater than length of replace string
      else if(strlen(substr)>strlen(replacestr)){   
//then replace substring with another string
          for(j=0,k=i;j<strlen(replacestr);j++,k++)   
            str[k] = replacestr[j];
//and remove extra characters
            for(j=1;j<=(strlen(substr)-strlen(replacestr));j++)   
              for(l=k-1;l<strlen(str);l++)
                str[l] = str[l+1];
      }
//length of substring is lesser than length of replace string
      else if(strlen(substr)<strlen(replacestr)){   
//then shift the charaters to right
          for(j=1;j<=(strlen(replacestr)-strlen(substr));j++) 
            for(l=strlen(str);l>k-1;l--)
              str[l+1] = str[l];
//and replace substring with another string
            for(j=0,k=i;j<strlen(replacestr)-1;j++,k++)
              str[k] = replacestr[j];
      }
      k = i;
    }
  }
  puts(str);
  return 0;
}

string = input()
substring = input()
replacestring = input()
if substring in string:
  string = string.replace(substring,replacestring)
print(string)

hello world how are you
world
developer
hello developer how are you