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 <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(s[j] > 'Z' && j == s.size() - 1) j++;
				else s[j] += 'a' - 'A';
				ret.push_back(s.substr(i, j - i));
				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>
using namespace std;

int main(){
  string str;
  char S;
  while(1){
    cin>>str;
    cin>>S;

    if(S=='X') break;

    else  if(S=='L'){
      if(str[0]>='A' && str[0]<='Z') str[0]+=32;
      for(int i=0;i<str.size();i++){
	if(str[i]=='_' && str[i+1]>='a' && str[i+1]<='z' ) str[i+1]-=32;
      }
      for(int i=0;i<str.size();i++){
	if(str[i]=='_') continue;
	else cout<<str[i];
      }
      cout<<endl;
    }
    
    else  if(S=='U') {
      if(str[0]>='a' && str[0]<='z') str[0]-=32;
    
    for(int i=0;i<str.size();i++){
    if(str[i]=='_'&& str[i+1]>='a' && str[i+1]<='z') str[i+1]-=32;
  }
  for(int i=0;i<str.size();i++){
	if(str[i]=='_') continue;
	else cout<<str[i];
      }
      cout<<endl;
      
    }
    
  else  if(S=='D'){
    for(int i=0;i<str.size();i++){
      if(str[i]>=65 && str[i]<=90){
	str[i]+=32;
      }
      if(i>0 && str[i+1]>=65 && str[i+1]<=90){
	cout<<str[i]<<"_";
      }
      else cout<<str[i];
    }
    cout<<endl;
  }
  }
  return 0;
  }

C++: #include<iostream>
#include<set>
#include<string>
#include<algorithm>
using namespace std;
string b[1000];
int main() {
	string a;
	char s;
	while (cin >> a >> s&&s != 'X') {
		bool l = false;
		for (int i = 0; i < a.length(); i++) {
			if (a[i] == '_') {
				l = true;
				break;
			}
		}
		int sum = 0;
		string h = "";
		if (l) {
			a += "_";
			for (int i = 0; i < a.length(); i++) {
				if (a[i] == '_') {
					b[sum] = h;
					sum++;
					h = "";
				}
				else
					h += a[i];
			}
		}
		else {
			if (a[0] <= 'Z'&&a[0] >= 'A')
				a[0] = (char)(int)a[0] - (int)('A' - 'a');
			a += 'A';
			for (int i = 0; i < a.length(); i++) {
				if (a[i] <= 'Z'&&a[i] >= 'A') {
					b[sum] = h;
					sum++;
					h = "";
					h += (char)((int)a[i] - (int)('A' - 'a'));
				}
				else
					h += a[i];
			}
		}
		switch (s) {
		case 'D':
			for (int i = 0; i < sum; i++) {
				if (i == 0) cout << b[i];
				else cout << "_" << b[i];
			}
			break;
		case 'U':
			for (int i = 0; i < sum; i++) {
				b[i][0] += ('A' - 'a');
				cout << b[i];
			}
			break;
		case 'L':
			for (int i = 0; i < sum; i++) {
				if(i!=0) b[i][0] += ('A' - 'a');
				cout << b[i];
			}
		}
		cout << endl;
	}
}