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 <cstdlib>
#include <cmath>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>

using namespace std;

#define rep(i,j) REP((i), 0, (j))
#define REP(i,j,k) for(int i=(j);(i)<(k);++i)
#define BW(a,x,b) ((a)<=(x)&&(x)<=(b))
#define F first
#define S second
#define INF 1 << 30

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

string name;
char type;

int main(){
  while(cin >> name >> type && type != 'X'){
    if(type == 'L'){
      if(name[0] >= 'A' && name[0] <= 'Z') name[0] += 32;
      rep(i, name.size()){
	if(name[i] != '_') continue;
	name.erase(name.begin()+i);
	name[i] -= 32;
      }
    }else if(type == 'U'){
      if(name[0] >= 'a' && name[0] <= 'z') name[0] -= 32;
      rep(i, name.size()){
	if(name[i] != '_') continue;
	name.erase(name.begin()+i);
	name[i] -= 32;
      }
    }else if(type == 'D'){
      if(name[0] >= 'A' && name[0] <= 'Z') name[0] += 32;
      rep(i, name.size()){
	if(name[i] >= 'a' && name[i] <= 'z') continue;
	name[i] += 32;
	name.insert(i, "_");
      }
    }

    cout << name << endl;
  }
  return 0;
}

C++: #include <iostream>
#include <string>
#include <sstream>
#include <algorithm>
#include <vector>
#include <utility>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
 
using namespace std;
 
#define rep(i,n) for(int i=0; i<(n); i++)
#define repc(i,s,e) for(int i=(s); i<(e); i++)
#define pb(n) push_back((n))
#define mp(n,m) make_pair((n),(m))
#define all(r) r.begin(),r.end()
#define fi first
#define se second
 
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vii;
typedef vector<ll> vl;
typedef vector<vl> vll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
 
 
const int INF = 10000000;

vector<string> split(const string &s, char c = ' '){
	vector<string> ret;
	stringstream ss(s);
	string t;
	while(getline(ss,t,c)){
		ret.pb(t);
	}
	return ret;
}

vector<string> get_word(string s){
	vector<string> ret;
	for(int i = 0; i < s.size(); i++){
		for(int j = i; j < s.size(); j++){
			if((s[j] >= 'A' && s[j] <= 'Z') || j == s.size() - 1){
				if(j == s.size() - 1) j++;
				ret.push_back(s.substr(i, j - i));
				if(j != s.size()) s[j] += 'a' - 'A';
				i = j - 1;
				break;
			}
		}
	}
	return ret;
}

string calc(string s, string t1, string t2){
	string ret;
	if(t1 == "U"){
		if(t2 == "L"){
			ret = s;
			ret[0] += 'a' - 'A';
		}
		else if(t2 == "D"){
			s[0] += 'a' - 'A';
			vector<string> v = get_word(s);
			// cout<<"  ";
			// for(int i = 0; i < v.size(); i++){
			// 	cout<<v[i];
			// }
			// cout<<endl;
			for(int i = 0; i < v.size(); i++){
				s = v[i];
				//s[0] += 'a' - 'A';
				if(ret.size() != 0) ret += "_";
				ret += s; 
			}
		}
		else ret = s;
	}
	else if(t1 == "L"){
		if(t2 == "U"){
			ret = s;
			ret[0] += 'A' - 'a';
		}
		else if(t2 == "D"){
			vector<string> v = get_word(s);
			for(int i = 0; i < v.size(); i++){
				s = v[i];
				//s[0] +='a' - 'A';
				if(ret.size() != 0) ret += "_";
				ret += s; 
			}
		}
		else ret = s;
	}
	else{
		if(t2 == "U"){
			vector<string> v = split(s, '_');
			for(int i = 0; i < v.size(); i++){
				s = v[i];
				s[0] += 'A' - 'a';
				ret += s;
			}
		}
		else if(t2 == "L"){
			vector<string> v = split(s, '_');
			for(int i = 0; i < v.size(); i++){
				s = v[i];
				if(i != 0) s[0] += 'A' - 'a';
				ret += s;
			}
		}
		else ret = s;
	}
	return ret;
}

int main(){
	string s, t;
	while(cin>>s>>t && t != "X"){
		if(s.find("_") != string::npos) cout<<calc(s, "D", t);
		else if(s[0] >= 'A' && s[0] <= 'Z') cout<<calc(s, "U", t);
		else cout<<calc(s,"L", t);
		cout<<endl;
	}
	
}

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

using namespace std;

int main(void) {
  string s;
  char c;
  while(1) {
    cin >> s >> c;
    if(c == 'X') break;
    switch(c) {
    case 'U':
      for(int i = 0; i < s.length(); i++) {
	if(i == 0) cout << (char)toupper(s[i]);
	else if(s[i] == '_') cout << (char)toupper(s[++i]);
	else cout << s[i];
      }
      break;
    case 'L':
      for(int i = 0; i < s.length(); i++) {
	if(i == 0) cout << (char)tolower(s[i]);
	else if(s[i] == '_') cout << (char)toupper(s[++i]);
	else cout << s[i];
      }
      break;
    case 'D':
      for(int i = 0; i < s.length(); i++) {
	if(i == 0) cout << (char)tolower(s[i]);
	else if(isupper(s[i])) cout << '_' << (char)tolower(s[i]);
	else cout << s[i];
      }
      break;
    }
    cout << endl;
  }

  return 0;
}