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 <string.h>
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;
}

C++: #include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int main(){
  char S[101],s;
  while(1){
    scanf("%s %c",S,&s);
    if(s=='X') break;
    else if(s=='L' || s=='U'){
      for(int i=0,l=strlen(S);i<l;i++){
	if(S[i]=='_' ||(s=='U'&&i==0&&S[i]>='a'&&S[i]<='z')){
	  if(i!=0)i++;
	  printf("%c",S[i]+ ('A'-'a'));
	}else  printf("%c",S[i]);
      }
    }else if(s=='D'){
      for(int i=1,l=strlen(S);i<l;i++){
	if(S[i]>='A'&&S[i]<='Z'){
	  printf("_%c",S[i]-('A'-'a'));
	}else{
	  printf("%c",S[i]);
	}
      }
    }
    puts("");
  }
}

C++: #include<iostream>
#include<vector>
using namespace std;

int main(){
  string s,t;

  for(;;){
    cin >> s;
    cin >> t;

    if(t=="X")break;

    vector<string> v;

    int pos=0;
    while(pos<s.size()){
      string tmp;
      if('A' <= s[pos] && s[pos]<='Z'){
	tmp += (char)(s[pos] + 32);
	pos++;
      }
      while(pos<s.size() && ('a'<= s[pos] && s[pos]<='z'))tmp += s[pos++];
      v.push_back(tmp);
      if(pos<s.size() && s[pos]=='_')pos++;
    }

    string ans;
    if(t=="U"){
      for(int i=0;i<(int)v.size();i++){
	v[i][0] = v[i][0]-32;
	ans += v[i];
      }
    }
    if(t=="L"){
      ans = v[0];
      for(int i=1;i<(int)v.size();i++){
	v[i][0] = v[i][0]-32;
	ans += v[i];
      }
    }
    if(t=="D"){
      ans = v[0];
      for(int i=1;i<(int)v.size();i++)ans += "_" + v[i];
    }
    cout << ans << endl;
  }
}

C++: #include<iostream>
#include<string>
#include<cctype>
#include<algorithm>
#include<functional>
using namespace std;
bool C(char c){return isupper(c);}
int main()
{
	string s;
	string::iterator i;
	char c;
	int x;
	while(cin>>s>>c,c-'X')
	{
		s[0]=toupper(s[0]);
		while(x=s.find('_'),x!=string::npos)
			s.erase(x,1),s[x]=toupper(s[x]);
		if(c-'U')s[0]=tolower(s[0]);
		if(c=='D')
			for(i=s.begin();(i=find_if(i,s.end(),ptr_fun(C)))!=s.end();++i)
				*i=tolower(*i),i=s.insert(i,'_');
		cout<<s<<endl;
	}
	return 0;
}

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

typedef ostringstream OSS;
typedef istringstream ISS;

typedef vector<int> VI;
typedef vector< VI > VVI;

typedef long long LL;

typedef pair<int, int> PII;
typedef vector<PII> VPII;

#define X first
#define Y second

string toUpperCamelCase(string s) {
	string res;

	for (int i = 0; i < (int)s.size(); i++) {
		if (i == 0) {
			res += toupper(s[i]);
		} else if (s[i] == '_') {
			res += toupper(s[i] + 1);
			i++;
		} else {
			res += s[i];
		}
	}

	return res;
}

string toLowerCamelCase(string s) {
	string res;

	for (int i = 0; i < (int)s.size(); i++) {
		if (i == 0) {
			res += tolower(s[i]);
		} else if (s[i] == '_') {
			res += toupper(s[i + 1]);
			i++;
		} else {
			res += s[i];
		}
	}

	return res;
}

string toSnakeCase(string s) {
	string res;

	for (int i = 0; i < (int)s.size(); i++) {
		if (i == 0) {
			res += tolower(s[i]);
		} else if (isupper(s[i])) {
			res += '_';
			res += tolower(s[i]);
		} else {
			res += s[i];
		}
	}

	return res;
}

int main(void) {
	string s;
	char c;

	while (true) {
		cin >> s >> c;
		if (c == 'X') break;

		switch (c) {
			case 'L':
				s = toLowerCamelCase(s);
				break;
			case 'U':
				s = toUpperCamelCase(s);
				break;
			case 'D':
				s = toSnakeCase(s);
				break;
		}

		cout << s << endl;
	}

	return 0;
}