C :

#include<stdio.h>
#include<string.h>
#define N 45
int main()
{
	char a[N],b[10];
	int i,flag=0,len,m=1,j;
	gets(a);
	gets(b);
	len=strlen(a);
	if(b[0]=='D'){
		for(i=0;a[i]!='\0';i++){
			if(a[i]==b[2]) flag=1;
		}
		if(flag==1){
			for(i=0;a[i]!='\0';i++){
				if(m==1 && a[i]==b[2]){
					m=0;
					continue;
				}
				else{
					printf("%c",a[i]);
				}
			}
		}
		else{
			printf("Not exist");
		}
		printf("\n");
	}
	else if(b[0]=='I'){
		for(i=len-1;i>=0;i--){
			if(a[i]==b[2]){
				flag=1;
				break;
			}
		}
		if(flag==1){		
			for(j=0;j<i;j++){
				printf("%c",a[j]);
			}
			printf("%c",b[4]);
			for(j=i;a[j]!='\0';j++){
				printf("%c",a[j]);
			}
		}
		else printf("Not exist");
		printf("\n");
	}
	else{
		for(i=0;a[i]!='\0';i++){
			if(a[i]==b[2]) flag=1;
		}
		if(flag==1){
			for(i=0;a[i]!='\0';i++){
				if(a[i]==b[2]){
					printf("%c",b[4]);
				}
				else{
					printf("%c",a[i]);
				}
			}
		}
		else{
			printf("Not exist");
		}
		printf("\n");
	}
} 

C++ :

#include<stdio.h>
#include<string.h>
int main()
{
	char a[110],b[110],ch,q,p;
	int i,count,j;
		for(i=0;;i++)
		{
			scanf("%c",&a[i]);
			if(a[i]=='.')
				break;
		}
		a[i+1]='\0';
		getchar();
		scanf("%c",&ch);
		getchar();
		if(ch=='D')
			scanf("%c",&q);
		else
			scanf("%c %c",&q,&p);
		getchar();
		if(ch=='D')
		{
			count=0,j=0;
			for(i=0;i<strlen(a);i++)
			{
				if(a[i]==q&&count==0)
				{
					count++;
				}
				else
				{
					b[j++]=a[i];
				}
			}
			b[j]='\0';
			if(count==0)
				printf("Not exist\n");
			else
			    printf("%s\n",b);
		}
		if(ch=='I')
		{
			count=0;j=strlen(a);
			for(i=strlen(a)-1;i>=0;i--)
			{
				if(a[i]==q&&count==0)
				{
					count++;
					b[j--]=a[i];
					b[j--]=p;
				}
				else
				{
					b[j--]=a[i];
				}
			}
			b[strlen(a)+1]='\0';
			if(count==0)
				printf("Not exist\n");
			else
			    printf("%s\n",b);
		}
		if(ch=='R')
		{
			count=0,j=0;
			for(i=0;i<strlen(a);i++)
			{
				if(a[i]==q)
				{
					b[j++]=p;
					count++;
				}
				else
					b[j++]=a[i];
			}
			b[j]='\0';
			if(count==0)
				printf("Not exist\n");
			else
				printf("%s\n",b);
		}
	return 0;
}

Pascal :

var
  s,t,s1:string;
  i:integer;
  a,b,c:char;
begin
  readln(s);
  s1:=s;
  readln(t);
  if t[1]='D' then begin
    i:=pos(t[3],s);
    delete(s,i,1);
  end else
    if t[1]='I' then begin
      for i:=length(s) downto 1 do if s[i]=t[3] then break;
        insert(t[5],s,i);
    end else
      begin
        for i:=1 to length(s) do if s[i]=t[3] then s[i]:=t[5];
      end;
  if s1=s then writeln('Not exist') else writeln(s);
end.

Java :

import java.util.Scanner;

import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		Scanner rd=new Scanner(System.in);
		int flag = 0;
		String str=rd.nextLine();
		String mingling=rd.next();
		String c=rd.next();
		StringEditor ed=new StringEditor(str,c);
		switch (mingling){
		
		case "D":
			ed.Del(str,c);
			break;
		case "I":
			String d = rd.next();
			ed.Insert(str, c, d);
			break;
		case "R":
			String d1 = rd.next();
			ed.Replace(str, c, d1);
			
			break;
		}
	}
}
class StringEditor
{
	private String fString;
	private String order;
	public StringEditor(String fString,String order)
	{
		this.fString=fString;
		this.order=order;
	}
	public void Del(String str,String b)
	{
		if(str.indexOf(b)!=-1)
		{
			str = str.replaceFirst(b,"");
			System.out.print(str);
		}
		else
			System.out.print("Not exist");
	}
	public void Insert(String str,String a1,String a2)
	{
		if(str.indexOf(a1)!=-1){
			
			int i=str.lastIndexOf(a1);
			StringBuilder sb=new StringBuilder(str);
			System.out.print(sb.insert(i, a2));
		}
		else
			System.out.println("Not exist");
	}
	public void Replace(String str,String a1,String a2)
	{
		if(str.indexOf(a1)!=-1)
		{
			String s=str.replaceAll(a1, a2);
			System.out.print(s);
		}
		else
			System.out.println("Not exist");
		
	}
}