Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

func configure(scanner *bufio.Scanner) {
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 1000005), 1000005)
}
func getNextString(scanner *bufio.Scanner) string {
	scanned := scanner.Scan()
	if !scanned {
		panic("scan failed")
	}
	return scanner.Text()
}
func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}
func getNextInt64(scanner *bufio.Scanner) int64 {
	i, _ := strconv.ParseInt(getNextString(scanner), 10, 64)
	return i
}
func getNextFloat64(scanner *bufio.Scanner) float64 {
	i, _ := strconv.ParseFloat(getNextString(scanner), 64)
	return i
}
func main() {
	fp := os.Stdin
	wfp := os.Stdout
	extra := 0
	if os.Getenv("I") == "IronMan" {
		fp, _ = os.Open(os.Getenv("END_GAME"))
		extra = 100
	}
	scanner := bufio.NewScanner(fp)
	configure(scanner)
	writer := bufio.NewWriter(wfp)
	defer func() {
		r := recover()
		if r != nil {
			fmt.Fprintln(writer, r)
		}
		writer.Flush()
	}()
	solve(scanner, writer)
	for i := 0; i < extra; i++ {
		fmt.Fprintln(writer, "-----------------------------------")
		solve(scanner, writer)
	}
}
func solve(scanner *bufio.Scanner, writer *bufio.Writer) {
	for s, c := getNextString(scanner), getNextString(scanner); c != "X"; s, c = getNextString(scanner), getNextString(scanner) {
		if strings.Index(s, "_") != -1 {
			fmt.Fprintln(writer, translate(strings.Split(s, "_"), c))
			continue
		}
		ss := make([]string, 0)
		for len(s) > 0 {
			index := nextupper(s[1:])
			ss = append(ss, s[:index+1])
			s = s[index+1:]
		}
		fmt.Fprintln(writer, translate(ss, c))
	}
}
func translate(ss []string, c string) string {
	switch c {
	case "U":
		return u(ss)
	case "L":
		return l(ss)
	case "D":
		return d(ss)
	}
	return ""
}

func nextupper(s string) int {
	n := len(s)
	for i := 0; i < n; i++ {
		if s[i:i+1] == strings.ToUpper(s[i:i+1]) {
			return i
		}
	}
	return n
}
func u(ss []string) string {
	n := len(ss)
	for i := 0; i < n; i++ {
		ss[i] = fmt.Sprintf("%s%s", strings.ToUpper(ss[i][0:1]), ss[i][1:])
	}
	return strings.Join(ss, "")
}
func l(ss []string) string {
	n := len(ss)
	ss[0] = strings.ToLower(ss[0])
	for i := 1; i < n; i++ {
		ss[i] = fmt.Sprintf("%s%s", strings.ToUpper(ss[i][0:1]), ss[i][1:])
	}
	return strings.Join(ss, "")
}
func d(ss []string) string {
	n := len(ss)
	for i := 0; i < n; i++ {
		ss[i] = strings.ToLower(ss[i])
	}
	return strings.Join(ss, "_")
}

C++: #include <iostream>
#include <string>
using namespace std;
int main(){
  int len,i,flag;
  string str,out;
  char type;
  while(1){
    flag=0;
    out="";
    cin >> str >> type;
    len=str.size();
    if(type=='X') break;
    if(type=='U'){
      for(i=0;i<len;i++){
	if(i==0&&str[i]>='a') out+=str[i]-'a'+'A';
	else{
	  if(str[i]=='_'){
	    out+=str[i+1]-'a'+'A';
	    i++;
	  }else out+=str[i];
	}
      }
    }
    else if(type=='L'){
	for(i=0;i<len;i++){
	  if(i==0&&str[i]<'a') out+=str[i]-'A'+'a';
	  else{
	    if(str[i]=='_'||str[i]<'a') flag=1;
	    if(str[i]=='_'){
	      out+=str[i+1]-'a'+'A';
	      i++;
	    }else out+=str[i];
	  }
	}
	if(flag==0) out[0]=out[0]-'a'+'A';
      }
      else if(type=='D'){
	for(i=0;i<len;i++){
	  if(i==0&&str[i]<'a') out+=str[i]-'A'+'a';
	  else{
	    if(str[i]<'a'){
	      out+='_';
	      out+=str[i]-'A'+'a';
	    }else out+=str[i];
	  }
	}
      }
    cout << out << endl;
  }
  return 0;
}

C++: #define _USE_MATH_DEFINES
#define INF 100000000
#include <iostream>
#include <sstream>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <stack>
#include <limits>
#include <map>
#include <string>
#include <cstring>
#include <set>
#include <deque>
#include <bitset>
#include <list>

using namespace std;

typedef long long ll;
typedef pair <int,int> P;

static const double EPS = 1e-8;

int main(){
	string str;
	char conv;
	while(cin>>str){
		cin >> conv;

		if(conv == 'X') break;
		//under score
		if(count(str.begin(),str.end(),'_')){
			if(conv == 'L'){
				//str[0] = 'A' + (str[0]-'a');
				string res="";
				for(int i=0;i+1<str.size();i++){
					if(str[i]=='_' && ('a' <= str[i+1] && str[i+1] <= 'z')){
						str[i+1]='A'+(str[i+1]-'a');
						i++;
					}
				}

				string::iterator it = remove(str.begin(),str.end(),'_');
				str.erase(it,str.end());
			}
			else if(conv == 'U'){
				str[0] = 'A' + (str[0]-'a');
				for(int i=0;i+1<str.size();i++){
					if(str[i]=='_' && ('a' <= str[i+1] && str[i+1] <= 'z')){
						str[i+1]='A'+(str[i+1]-'a');
					}
				}

				remove(str.begin(),str.end(),'_');
			}
		}
		//Upper CamelCase
		else if('A' <= str[0] && str[0] <= 'Z'){
			if(conv == 'L'){
				str[0] = 'a' + (str[0]-'A');
			}
			else if(conv == 'D'){
				string res="";
				str[0] = 'a' + (str[0]-'A');

				res.push_back(str[0]);
				for(int i=1;i<str.size();i++){
					if('A' <= str[i] && str[i] <= 'Z'){
						res.push_back('_');
						res.push_back('a' + (str[i]-'A'));
					}
					else{
						res.push_back(str[i]);
					}
				}

				str = res;
			}

		}
		//Lower CamelCase
		else{
			if(conv == 'U'){
				str[0] = 'A' + (str[0]-'a');
			}
			else if(conv == 'D'){
				string res="";
				str[0] = 'a' + (str[0]-'a');
				res.push_back(str[0]);
				for(int i=1;i<str.size();i++){
					if('A' <= str[i] && str[i] <= 'Z'){
						res.push_back('_');
						res.push_back('a' + str[i]-'A');
					}
					else{
						res.push_back(str[i]);
					}
				}
				str = res;
			}
		}

		cout << str << endl;
	}
}

C++: #include <cctype>
#include <string>
#include <iostream>

using namespace std;

string LowerCamelCaseDiv2(string S)
{
    string T;
    
    for(int i = 0; i < S.size(); i++)
    {
        if(i == 0)
        {
            T += tolower(S[i]);
        }  
        else if(S[i - 1] == '_')
        {
            T += toupper(S[i]);
        }
        else if(S[i] != '_')
        {
            T += S[i];
        }
    }
    
    return T;
}

string UpperCamelCaseDiv2(string S)
{
    string T;
    
    for(int i = 0; i < S.size(); i++)
    {
        if(i == 0)
        {
            T += toupper(S[i]);
        }  
        else if(S[i - 1] == '_')
        {
            T += toupper(S[i]);
        }
        else if(S[i] != '_')
        {
            T += S[i];
        }
    }
    
    return T;
}

string UnderLineDiv2(string S)
{
    string T;
    
    for(int i = 0; i < S.size(); i++)
    {
        if(i != 0 && isupper(S[i]))
        {
            T += '_';
        }
        
        T += tolower(S[i]);
    }
    
    return T;
}

int main()
{
    string S, T;
    
    while(true)
    {
        cin >> S >> T;
        
        if(T == "X") { break; }
        
        if(T == "L") { cout << LowerCamelCaseDiv2(S) << endl; }
        if(T == "U") { cout << UpperCamelCaseDiv2(S) << endl; }
        if(T == "D") { cout << UnderLineDiv2(S) << endl; }
    }
    
    return 0;
}

C++: #include<iostream>
#include<string>
using namespace std;

void smallToLarge( char& a )
{
    if( 'a' <= a && a <= 'z' )
        a -= ( 'a' - 'A' );
}

void LargeToSmall( char& a )
{
    if( 'A' <= a && a <= 'Z' )
        a += ( 'a' - 'A' );
}

int main()
{
    string s;
    char c;

    while( cin >> s >> c )
    {
        int a = s.size();

        if( c == 'X' )
            break;
        else if( c == 'U' || c == 'L' )
        {
            c == 'U' ? smallToLarge(s[0]) : LargeToSmall(s[0]);
            for( int i = 0; i < a; i++ )
                if( s[i] == '_' )
                    smallToLarge(s[i+1]);
                else
                    cout << s[i];
        }
        else
        {
            LargeToSmall(s[0]);
            for( int i = 0; i < a; i++ )
            {
                if( 'A' <= s[i] && s[i] <= 'Z' )
                {
                    cout << '_';
                    LargeToSmall(s[i]);
                }
                cout << s[i];
            }
        }
        cout << endl;
    }
    return 0;
}