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 <vector>
#include <string>
#include <algorithm>

using namespace std;

int main(){

	string str;
	while(1){
		getline(cin,str);
		string name = str.substr(0,str.size()-2);
		char type = str[str.size()-1];
		if(type=='L'){
			if(name[0] >= 'a' && name[0] <= 'z'){
				for(int i = 0; i < name.size(); i++){
					if(name[i] == '_'){
						cout << (char)(name[i+1]-'a'+'A') << flush;
						i++;
					}
					else{
						cout << name[i] << flush;
					}
				}
				cout<<endl;
			}
			else{
				cout << (char)(name[0]-'a'+'A') << flush;
				for(int i = 1; i < name.size(); i++){
					cout << name[i]<<flush;
				}
				cout<<endl;
			}
		}
		else if(type=='U'){
			if(name[0] >= 'a' && name[0] <= 'z'){
				cout << (char)(name[0]-'a'+'A') << flush;
				for(int i = 1; i < name.size(); i++){
					if(name[i] == '_'){
						cout << (char)(name[i+1]-'a'+'A') << flush;
						i++;
					}
					else{
						cout << name[i]<<flush;
					}
				}
				cout<<endl;
			}
			else{
				cout << name[0] << flush;
				for(int i = 1; i < name.size(); i++){
					cout << name[i]<<flush;
				}
				cout<<endl;
			}
		}
		else if(type=='D'){
			if(name[0] >= 'a' && name[0] <= 'z'){
				for(int i = 0; i < name.size(); i++){
					if(name[i] >= 'A' && name[i] <= 'Z'){
						cout <<'_'<<flush;
						cout << (char)(name[i]+'a'-'A') << flush;
						i++;
					}
					else{
						cout << name[i] << flush;
					}
				}
				cout<<endl;
			}
			else{
				cout << (char)(name[0]-'A'+'a') << flush;
				for(int i = 1; i < name.size(); i++){
					if(name[i] >= 'A' && name[i] <= 'Z'){
						cout <<'_'<<flush;
						cout << (char)(name[i] - 'A'+'a') << flush;
					}
					else
						cout << name[i]<<flush;
				}
				cout << endl;
			}
		}
		else
			break;
		//cout<<endl;
	}

	return 0;
}

C++: #include <iostream>
#include <complex>
#include <sstream>
#include <string>
#include <algorithm>
#include <deque>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <vector>
#include <set>
#include <limits>
#include <cstdio>
#include <cctype>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <ctime>
using namespace std;
#define REP(i, j) for(int i = 0; i < (int)(j); ++i)
#define FOR(i, j, k) for(int i = (int)(j); i < (int)(k); ++i)
#define SORT(v) sort((v).begin(), (v).end())
#define REVERSE(v) reverse((v).begin(), (v).end())
typedef complex<double> P;

string ULtoD(string s){
  stringstream ss;
  ss << (char)tolower(s[0]);
  FOR(i, 1, s.length()){
    if(isupper(s[i])) ss << '_' << (char)tolower(s[i]);
    else ss << s[i];
  }
  return ss.str();
}

string DtoUL(string s, char t){
  stringstream ss;
  ss << (char)(t == 'U' ? toupper(s[0]) : s[0]);
  FOR(i, 1, s.length()){
    if(s[i] == '_') ss << (char)toupper(s[++i]);
    else ss << s[i];
  }
  return ss.str();
}

int main() {
  string s; char t;
  while(cin >>s >>t && s != "EndOfInput"){
    char bt;
    if(isupper(s[0])) bt = 'U';
    else if(s.find('_') != string::npos) bt = 'D';
    else bt = 'L';
    if(bt == 'U'){
      if(t == 'L') s[0] = tolower(s[0]);
      if(t == 'D') s = ULtoD(s);
    } else if(bt == 'L'){
      if(t == 'U') s[0] = toupper(s[0]);
      if(t == 'D') s = ULtoD(s);
    } else s = DtoUL(s, t);
    cout <<s <<endl;
  }
  return 0;
}

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

string solve(string s, char c){
    if('A' <= s[0] && s[0] <= 'Z')
        s[0] += 'a' - 'A';
    s += '_';
    int n = s.size();

    int j = 0;
    vector<string> vs;
    for(int i=0; i<n; ++i){
        if('A' <= s[i] && s[i] <= 'Z'){
            s[i] += 'a' - 'A';
            vs.push_back(s.substr(j, i-j));
            j = i;
        }else if(s[i] == '_'){
            vs.push_back(s.substr(j, i-j));
            j = i+1;
        }
    }

    int m = vs.size();
    if(c == 'U')
        vs[0][0] += 'A' - 'a';
    s = vs[0];

    for(int i=1; i<m; ++i){
        if(c == 'D'){
            s += '_' + vs[i];
        }else{
            vs[i][0] += 'A' - 'a';
            s += vs[i];
        }
    }
    return s;
}

int main()
{
    for(;;){
        string s;
        char c;
        cin >> s >> c;
        if(s == "EndOfInput")
            return 0;

        cout << solve(s, c) << endl;
    }
}