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 <bits/stdc++.h>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
const double PI = acos(-1);
template<class T>constexpr T INF() { return ::std::numeric_limits<T>::max(); }
template<class T>constexpr T HINF() { return INF<T>() / 2; }
template <typename T_char>T_char TL(T_char cX) { return tolower(cX); };
template <typename T_char>T_char TU(T_char cX) { return toupper(cX); };
template<typename T>vector<T> make_v(size_t a){ return vector<T>(a); }
template<typename T,typename... Ts>auto make_v(size_t a, Ts... ts){ return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...)); }
template<typename T,typename V>typename enable_if<is_class<T>::value==0>::type fill_v(T &t, const V &v){ t = v; }
template<typename T,typename V>typename enable_if<is_class<T>::value!=0>::type fill_v(T &t, const V &v){ for(auto &e : t) fill_v(e, v); }
const int vy[] = { -1, -1, -1, 0, 1, 1, 1, 0 }, vx[] = { -1, 0, 1, 1, 1, 0, -1, -1 };
const int dx[4] = { 0,1,0,-1 }, dy[4] = { 1,0,-1,0 };
int popcnt(unsigned long long n) { int cnt = 0; for (int i = 0; i < 64; i++)if ((n >> i) & 1)cnt++; return cnt; }
int d_sum(LL n) { int ret = 0; while (n > 0) { ret += n % 10; n /= 10; }return ret; }
int d_cnt(LL n) { int ret = 0; while (n > 0) { ret++; n /= 10; }return ret; }
LL gcd(LL a, LL b) { if (b == 0)return a; return gcd(b, a%b); };
LL lcm(LL a, LL b) { LL g = gcd(a, b); return a / g*b; };
# define ALL(qpqpq)           (qpqpq).begin(),(qpqpq).end()
# define UNIQUE(wpwpw)        sort(ALL((wpwpw)));(wpwpw).erase(unique(ALL((wpwpw))),(wpwpw).end())
# define LOWER(epepe)         transform(ALL((epepe)),(epepe).begin(),TL<char>)
# define UPPER(rprpr)         transform(ALL((rprpr)),(rprpr).begin(),TU<char>)
# define FOR(i,tptpt,ypypy)   for(LL i=(tptpt);i<(ypypy);i++)
# define REP(i,upupu)         FOR(i,0,upupu)
# define INIT                 std::ios::sync_with_stdio(false);std::cin.tie(0)

string s;
char c;

bool isLower(char x){
    if('a' <= x && x <= 'z')return true;
    return false;
}

char toUpper(char x){
    return (char)(x - 32);
}

char toLower(char x){
    return (char)(x + 32);
}

string fL(string t){
    string ret = "";
    REP(i, (int)t.size()){
        if(0 < i && t[i - 1] == '_'){
            ret = ret + toUpper(t[i]);
        }
        else if(t[i] != '_')ret = ret + t[i];
    }
    if(!isLower(ret[0])){
        ret[0] = tolower(ret[0]);
    }
    return ret;
}

string fU(string t){
    string ret = "";
    REP(i, (int)t.size()){
        if(0 < i && t[i - 1] == '_'){
            ret = ret + toUpper(t[i]);
        }
        else if(t[i] != '_')ret = ret + t[i];
    }
    if(isLower(ret[0])){
        ret[0] = toUpper(ret[0]);
    }
    return ret;
}

string fD(string t){
    string ret = "";
    if(!isLower(t[0])){
        t[0] = tolower(t[0]);
    }
    REP(i, (int)t.size()){
        if(!isLower((char)(t[i]))){
            ret = ret + (string)("_");
            if(!isLower(t[i])){
                t[i] = toLower(t[i]);
            }
        }
        ret = ret + t[i];
    }
    if(!isLower(ret[0])){
        ret[0] = tolower(ret[0]);
    }
    return ret;
}

int main(){
    while(cin >> s >> c && c != 'X'){
        if(c == 'U'){
            cout << fU(s) << endl;
        }else if(c == 'L'){
            cout << fL(s) << endl;
        }else{
            bool flg = false;
            REP(i, (int)s.size()){
                if(s[i] == '_'){
                    flg = true;
                }
            }
            if(flg)cout << s << endl;
            else cout << fD(s) << endl;
        }
    }
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <string.h>
#include <string>
#include <algorithm>
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] == '_'){
	    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;
}