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;
  string str,out;
  char type;
  while(1){
    out="";
    cin >> str >> type;
    len=str.size();
    if(type=='X') break;
    if(type=='U'){
      for(i=0;i<len;i++){
	if(i==0) 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]=='_'){
	    out+=str[i+1]-'a'+'A';
	    i++;
	  }else out+=str[i];
	}
      }
    }
    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++: #include <iostream>
#include <cctype>
#include <vector>

using namespace std;

int len;

string UpperCamelCase(vector<string> v){
  string res = "";

  for(int i = 0 ; i < len ; i++){
    if(islower(v[i][0])){
      v[i][0] = toupper(v[i][0]);
    }
    res += v[i];
  }

  return res;
}

string LowerCamelCase(vector<string> v){
  string res = v[0];

  for(int i = 1 ; i < len ; i++){
    if(islower(v[i][0])){
      v[i][0] = toupper(v[i][0]);
    }
    res += v[i];
  }

  return res;
}

string Under(vector<string> v){
  string res = v[0];

  for(int i = 1 ; i < len ; i++){
    res += '_' + v[i];
  }
  
  return res;
}

int main(){
  string s;
  char order;

  while(cin >> s >> order, order != 'X'){
    if(isupper(s[0])){
      s[0] = tolower(s[0]);
    }
    s += '_';

    vector<string> v;
    int pos = 0;
    len = (int)s.size();

    for(int i = 0 ; i < len ; i++){
      if(isupper(s[i])){
	s[i] = tolower(s[i]);
	v.push_back(s.substr(pos,i-pos));
	pos = i;
      }else if(s[i] == '_'){
	v.push_back(s.substr(pos,i-pos));
	pos = i + 1;
      }
    }

    len = (int)v.size();
    
    if(order == 'U'){
      cout << UpperCamelCase(v) << endl;
    }else if(order == 'L'){
      cout << LowerCamelCase(v) << endl;
    }else{
      cout << Under(v) << endl;
    }
  }

  return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <string.h>
#include <string>
#include <algorithm>
#include <cctype>
using namespace std;
int main(){
  char op;
  string str;
  while(cin >> str >> op, op != 'X'){
    int flag[str.size()];
    memset(flag, 0, sizeof(flag));
    for(int i = 0 ; i < (int)str.size() ; i++){
      if('A' <= str[i] && str[i] <= 'Z') flag[i] = 1;
      if(str[i] == '_') flag[i] = 2;
    }

    if(op != 'D'){
      if(op == 'U'){
	if('a' <= str[0] && str[0] <= 'z') str[0] -= 0x20; //toupper(str[0]);
	for(int i = 0 ; i < (int)str.size() ; i++){
	  
	  if(str[i] == '_'){
	    str[i] = 0;
	    if('a' <= str[i+1] && str[i+1] <= 'z') str[i+1] -= 0x20; //toupper(str[i+1]);
	  }
	}
      }
      
      if(op == 'L'){
	if('A' <= str[0] && str[0] <= 'Z') str[0] += 0x20;  //tolower(str[0]);
	for(int i = 0 ; i < (int)str.size() ; i++){
	  if(flag[i] == 2 && str[i] == '_'){
	    str[i] = 0;
	    if('a' <= str[i+1] && str[i+1] <= 'z') str[i+1] -= 0x20;// toupper(str[i+1]);
	  }
	}
      }
      cout << str;
    }
    
    if(op == 'D'){
      for(int i = 0 ; i < (int)str.size() ; i++){
	if('A' <= str[i] && str[i] <= 'Z') str[i] += 0x20;//tolower(str[i]);
	if(flag[i] == 1 && i != 0){
	  cout << '_';
	}
	cout << str[i];
      }
    }  
    cout << endl;   
  }
  return 0;
}

C++: #include<iostream>
#include<string>
using namespace std;
string S; char C;
int main() {
	while (true) {
		cin >> S >> C; string D;
		if (C == 'X')break;
		if (C == 'L') {
			for (int i = 0; i < S.size(); i++) {
				if (S[i] == '_') {
					i++; if (S[i] <= 'Z')D += S[i];
					else D += (char)(S[i] - 32);
				}
				else {
					D += S[i];
				}
			}
			if (D[0] <= 'Z')D[0] += 32;
		}
		if (C == 'U') {
			for (int i = 0; i < S.size(); i++) {
				if (S[i] == '_') {
					i++; if (S[i] <= 'Z')D += S[i];
					else D += (char)(S[i] - 32);
				}
				else {
					D += S[i];
				}
			}
			if (D[0] > 'Z')D[0] -= 32;
		}
		if (C == 'D') {
			for (int i = 0; i < S.size(); i++) {
				if (S[i] <= 'Z' && i >= 1) { D += '_'; }
				if (S[i] <= 'Z') { D += (char)(S[i] + 32); }
				else { D += S[i]; }
			}
		}
		cout << D << endl;
	}
	return 0;
}