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>
using namespace std;
int main(){
  char op;
  string str;
  while(cin >> str >> op, op != 'X'){
    bool flag[str.size()];
    memset(flag, 0, sizeof(flag));
    for(int i = 0 ; i < str.size() ; i++){
      if('A' <= str[i] && str[i] <= 'Z' ||
	 str[i] == '_') flag[i] = 1;
    }

    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] == 1){
	  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>
#include <vector>

using namespace std;

string convL(vector<string> vec)
{
  string str = vec[0];

  for(int i = 1; i < vec.size(); i++){
    string st = vec[i];
    str += (char)('A'+(int)(st[0]-'a'));
    for(int j = 1; j < st.size(); j++){
      str += st[j];
    }
  }

  return str;
}

string convU(vector<string> vec)
{
  string str = "";

  for(int i = 0; i < vec.size(); i++){
    string st = vec[i];
    str += (char)('A'+(int)(st[0]-'a'));
    for(int j = 1; j < st.size(); j++){
      str += st[j];
    }
  }

  return str;
}

string convD(vector<string> vec)
{
  string str = vec[0];

  for(int i = 1; i < vec.size(); i++){
    str += ("_" + vec[i]);
  }

  return str;
}

vector<string> split(string str)
{
  vector<string> vec;
  string s = "";

  for(int i = 0; i < str.size(); i++){
    if(str[i] == '_'){
      vec.push_back(s);
      s = "";
    } else if(str[i] >= 'a' && str[i] <= 'z'){
      s += str[i];
    } else {
      if(s.size() > 0){
	vec.push_back(s);
      }
      s = "";
      s += (char)('a'+(int)(str[i]-'A'));
    }
  }
  if(s.size() > 0){
    vec.push_back(s);
  }

  return vec;
}

int main()
{
  string s1, s2;

  while(cin>>s1>>s2){
    if(s2 == "X"){
      break;
    }
    vector<string> vec = split(s1);

    if(s2 == "L"){
      s1 = convL(vec);
    } else if(s2 == "U"){
      s1 = convU(vec);
    } else {
      s1 = convD(vec);
    }

    cout << s1 << endl;
  }

  return 0;
}

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

#define to 'A'-'a'
#define big(x) (x) >= 'A' && (x) <= 'Z'

int main(){
    string line;
    char s;
    while(1){
        cin >> line >> s;
        if(s == 'X') break;
        if(s == 'U' || s == 'L'){
            for(int i=0;i<line.size();i++){
                if(i == 0 && s == 'U') line[i] += to;
                if(line[i] == '_') line[i+1] += to;
            }
            while(line.find("_") != string::npos) line.erase(line.find("_"),1);
        }
        else {
            for(int i=0;i<line.size();i++){
                if(i == 0 && big(line[i])) line[i] -= to;
                else if(big(line[i])) {
                    line[i] -= to;
                    line.insert(i,"_");
                }
            }
        }
        cout << line << endl;
    }
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <cmath>
#include <cstdio>
#include <istream>
#include <sstream>
#include <iomanip>
#include <iterator>
#include <climits>
using namespace std;

typedef ostringstream OSS;
typedef istringstream ISS;

typedef vector<int> VI;
typedef vector< VI > VVI;

typedef long long LL;

typedef pair<int, int> PII;
typedef vector<PII> VPII;

#define X first
#define Y second

string toUpperCamelCase(string s) {
	string res;

	for (int i = 0; i < (int)s.size(); i++) {
		if (i == 0) {
			res += toupper(s[i]);
		} else if (s[i] == '_') {
			res += toupper(s[i + 1]);
			i++;
		} else {
			res += s[i];
		}
	}

	return res;
}

string toLowerCamelCase(string s) {
	string res;

	for (int i = 0; i < (int)s.size(); i++) {
		if (i == 0) {
			res += tolower(s[i]);
		} else if (s[i] == '_') {
			res += toupper(s[i + 1]);
			i++;
		} else {
			res += s[i];
		}
	}

	return res;
}

string toSnakeCase(string s) {
	string res;

	for (int i = 0; i < (int)s.size(); i++) {
		if (i == 0) {
			res += tolower(s[i]);
		} else if (isupper(s[i])) {
			res += '_';
			res += tolower(s[i]);
		} else {
			res += s[i];
		}
	}

	return res;
}

int main(void) {
	string s;
	char c;

	while (true) {
		cin >> s >> c;
		if (c == 'X') break;

		switch (c) {
			case 'L':
				s = toLowerCamelCase(s);
				break;
			case 'U':
				s = toUpperCamelCase(s);
				break;
			case 'D':
				s = toSnakeCase(s);
				break;
		}

		cout << s << endl;
	}

	return 0;
}