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

void toUpperString(string& upper_str, const string& src_str)
{
  bool is_under_score = false;

  if (src_str[0] >= 0x61 && src_str[0] <= 0x7A) {
    upper_str += src_str[0] - 0x20;
  } else {
    upper_str += src_str[0];
  }
  
  for (int i = 1; i != src_str.size(); i++) {
    if (src_str[i] == '_') {
      is_under_score = true;
      continue;
    }
    if (!is_under_score) {
      upper_str += src_str[i];
    } else {
      upper_str += src_str[i] - 0x20;
      is_under_score = false;
    }
  }
}

void toLowerString(string& lower_str, const string& src_str)
{
  bool is_under_score = false;

  if (src_str[0] >= 0x41 && src_str[0] <= 0x5A) {
    lower_str += src_str[0] + 0x20;
  } else {
    lower_str += src_str[0];
  }

  for (int i = 1; i != src_str.size(); i++) {
    if (src_str[i] == '_') {
      is_under_score = true;
      continue;
    }
    if (!is_under_score) {
      lower_str += src_str[i];
    } else {
      lower_str += src_str[i] - 0x20;
      is_under_score = false;
    }
  }
}

void toSnakeString(string& snake_str, const string& src_str)
{
  if (src_str[0] >= 0x41 && src_str[0] <= 0x5A) {
    snake_str += src_str[0] + 0x20;
  } else {
    snake_str += src_str[0];
  }

  for (int i = 1; i != src_str.size(); i++) {
    if (src_str[i] == '_' ||
        (src_str[i] >= 0x61 && src_str[i] <= 0x7A)) {
      snake_str += src_str[i];
    } else {
      snake_str += '_';
      snake_str += src_str[i] + 0x20;
    }
  }
}

  
int main()
{
  string name;
  string output;
  char type;

  while (1) {
    cin >> name >> type;
    cin.ignore();

    output.clear();
    if (type == 'U') {
      toUpperString(output, name);
      cout << output << endl;
    } else if (type == 'L') {
      toLowerString(output, name);
      cout << output << endl;
    } else if (type == 'D') {
      toSnakeString(output, name);
      cout << output << endl;
    } else if (type == 'X') {
      break;
    }
  }
}