A Sub Expression Parser - useful to extract the expressions from regex groupings where the regex is a series of "OR" clauses that translate to options in a drop down etc.

gistfile1.txt

package optionsparser;

import java.io.IOException;
import java.util.Scanner;
import java.util.Stack;


public class SubExpressionParser {

	public static void main(String[] args) {
		String input = "(^a bc$)|(xyz)|(123)";
		String output;
		InToPost theTrans = new InToPost(input);
		output = theTrans.doTrans();
		System.out.println("Postfix is " + output + '\n');
		System.out.println("evaluated: " + Postfix.postfixEvaluate(output));
	}
	
	public static class InToPost {
		private Stack theStack;
		private String input;
		private String output = "";

		public InToPost(String in) {
			input = in;
			int stackSize = input.length();
			theStack = new Stack(stackSize);
		}

		public String doTrans() {
			for (int j = 0; j < input.length(); j++) {
				char ch = input.charAt(j);
				switch (ch) {
				case '|':
					gotOper(ch, 1);
					break;
				case '(':
					theStack.push(ch);
					break;
				case ')':
					gotParen(ch);
					output = output + ",;.;,";
					break;
				default:
					output = output + ch;
					break;
				}
			}
			while (!theStack.isEmpty()) {
				output = output + theStack.pop() + ",;.;,";
			}
			System.out.println(output);
			return output;
		}

		public void gotOper(char opThis, int prec1) {
			while (!theStack.isEmpty()) {
				char opTop = theStack.pop();
				if (opTop == '(') {
					theStack.push(opTop);
					break;
				} else {
					int prec2;
					if (opTop == '|')// || opTop == '-')
						prec2 = 1;
					else
						prec2 = 2;
					if (prec2 < prec1) {
						theStack.push(opTop);
						break;
					} else
						output = output + opTop + ",;.;,";
				}
			}
			theStack.push(opThis);
		}

		public void gotParen(char ch) {
			while (!theStack.isEmpty()) {
				char chx = theStack.pop();
				if (chx == '(')
					break;
				else
					output = output + chx;
			}
		}

		public static void main(String[] args) throws IOException {
			String input = "(^a bc$)|(xyz)|(123)";
			String output;
			InToPost theTrans = new InToPost(input);
			output = theTrans.doTrans();
			System.out.println("Postfix is " + output + '\n');
			System.out.println("evaluated: " + Postfix.postfixEvaluate(output));
		}

		class Stack {
			private int maxSize;
			private char[] stackArray;
			private int top;

			public Stack(int max) {
				maxSize = max;
				stackArray = new char[maxSize];
				top = -1;
			}

			public void push(char j) {
				stackArray[++top] = j;
			}

			public char pop() {
				return stackArray[top--];
			}

			public char peek() {
				return stackArray[top];
			}

			public boolean isEmpty() {
				return (top == -1);
			}
		}
	}
	public static class Postfix {
	    public static void main(String[] args) {
	        System.out.println(postfixEvaluate("^a bc$,;.;,xyz,;.;,|,;.;,123,;.;,|,;.;,"));       
	    }
	    
	    public static String postfixEvaluate(String exp) {
		 	Stack<String> s = new Stack<String> ();
			Scanner tokens = new Scanner(exp).useDelimiter("\\s*,;.;,\\s*");;
			
			while (tokens.hasNext()) {
				if (!tokens.hasNext("\\|")) {
					s.push(tokens.next());
				} else {
					String num2 = s.pop();
					String num1 = s.pop();
					String op = tokens.next();
					
					s.push(num2 + ",;.;," + num1);
				}
			}
			return s.pop();
	    }
	}
}