From camelCase to UNDER_LINE convention converter. Another regex fun...

gistfile1.java

package com.blog.oguzhan.charutil;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class ConventionConverter {
  
	public static void main(String[] args) {
		String[] output = camelCaseToUnderline(new String[]{"abc","aBc","abC","abCD"});
	
		for (String out : output) {
			System.out.println(out);
		}
	}
	
	public static final String[] camelCaseToUnderline(String[] words) {
		if (words != null && words.length > 0) {
			String[] outputWords = new String[words.length];
			int index = 0;
			for (String word : words) {
				outputWords[index] = camelCaseToUnderline(word);
				index++;
			}
			
			return outputWords;
		}
		else {
			throw new IllegalArgumentException("INVALID INPUT");
		}
	}
	
	public static final String camelCaseToUnderline(String word) {
		if (word != null) {
			StringBuffer outputWord = new StringBuffer(word.length());
			
			String lowPartPattern = "[a-z]+";
			String highPartPattern = "[A-Z][a-z]*";
			String camelCasePattern = "[a-z]+([A-Z]{0,1}[a-z]*)*";
			Pattern pattern = Pattern.compile(camelCasePattern);
			Pattern lowPattern = Pattern.compile(lowPartPattern);
			Pattern highPattern = Pattern.compile(highPartPattern);

			Matcher matcher = pattern.matcher(word);
			while(matcher.find()) {
				// find next camel case part 
				String found = matcher.group();
				
				// grab the low part of the camel case
				Matcher lowMatcher = lowPattern.matcher(found);
				if(lowMatcher.find()) {
					String lowPart = lowMatcher.group();
					lowPart = lowPart.toUpperCase();					
					outputWord.append(lowPart);
				}
			
				// grab the high part of the camel case
				Matcher highMatcher = highPattern.matcher(found);
				while(highMatcher.find()) {
					String hightPart = highMatcher.group();
					hightPart = hightPart.toUpperCase();
					outputWord.append("_");
					outputWord.append(hightPart);
				}
			}
			
			return outputWord.toString();
		}
		else {
			throw new IllegalArgumentException("INVALID INPUT");
		}
	}
		
}