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 <functional>
#include <algorithm>
#include <iostream>
#include <numeric>
#include <iomanip>
#include <utility>
#include <cstdlib>
#include <sstream>
#include <bitset>
#include <vector>
#include <cstdio>
#include <ctime>
#include <queue>
#include <deque>
#include <cmath>
#include <stack>
#include <list>
#include <map>
#include <set>

using namespace std;

typedef vector<int> vi;
typedef pair<int,int> pii;
typedef long long ll;

#define dump(x)  cerr << #x << " = " << (x) << endl
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(a)  (a).begin(),(a).end()
#define pb push_back


string sol(string s){
    string ret="";
    
    rep(i,s.size()){
        if(i!=0 && isupper(s[i]) && s[i]!='_'){
            ret+='_';
            ret+=tolower(s[i]);
        }
        else{
            ret+=tolower(s[i]);
        }
    }
    return ret;
}





int main() {
    string s;
    char type;
    
    while(cin>>s>>type){
        if(type=='X')break;
        
        s=sol(s);
        
        if(type=='D')cout<<s<<endl;
        else if(type=='L'){
            string ans="";
            rep(i,s.size()){
                if(s[i]=='_'){
                    ans+=toupper(s[i+1]);
                    i++;
                }
                else ans+=tolower(s[i]);
            }
            cout<<ans<<endl;
            
        }
        else if(type=='U'){
            string ans="";
            rep(i,s.size()){
                if(i==0){
                    ans+=toupper(s[i]);
                }
                else if(s[i]=='_'){
                    ans+=toupper(s[i+1]);
                    i++;
                }
                else ans+=tolower(s[i]);
            }
            cout<<ans<<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' || C == 'D') {
			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 {
					if (S[i] > 'Z')D += S[i];
					else D += (char)(S[i] + 32);
				}
			}
			if (S[0] <= 'Z')S[0] += 32;
		}
		if (C == 'U') {
			if (S[0] > 'Z')S[0] -= 32;
		}
		if (C == 'D') {
			for (int i = 0; i < S.size(); i++) {
				if (S[i] <= 'Z' && i >= 1) { D += '_'; }
				D += S[i];
			}
		}
		cout << D << endl;
	}
	return 0;
}

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

using namespace std;

string UOP(string str){
  string ans="";
  if('a' <= str[0] && str[0] <= 'z') str[0] = toupper(str[0]);
  for(int i = 0 ; i < str.size() ; i++){
    if(str[i] == '_'){
      if('a' <= str[i+1] && str[i+1] <= 'z') str[i+1] = toupper(str[i+1]);
    }
    else ans += str[i];
  }
  return ans;
}

string LOP(string str){
  string ans="";
  if('A' <= str[0] && str[0] <= 'Z') str[0] = tolower(str[0]);
  for(int i = 0 ; i < str.size() ; i++){
    if(str[i] == '_'){
      if('a' <= str[i+1] && str[i+1] <= 'z') str[i+1] = toupper(str[i+1]);
    }
    else ans += str[i];
  }
  return ans;
}

string DOP(string str){
  string ans="";
  for(int i = 0 ; i < str.size() ; i++){
    if('A' <= str[i] && str[i] <= 'Z') str[i] = tolower(str[i]);
    ans += str[i];
    if('A' <= str[i+1] && str[i+1] <= 'Z') ans += '_';
  }
  return ans;
}

int main(){
  string str;
  char op;
  while(cin >> str >> op, op != 'X'){
    if(op == 'U'){
      cout << UOP(str) << endl;
    }
    else if(op == 'L'){
      cout << LOP(str) << endl;
    }
    else if(op == 'D'){
      cout << DOP(str) << 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 << (char)tolower(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 && t != 'X'){
    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 if(t != 'D') s = DtoUL(s, t);
    cout <<s <<endl;
  }
  return 0;
}