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With inspiration from other libraries
There is a newer version: 2.12.15
/**
 * Logback: the reliable, generic, fast and flexible logging framework.
 * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
 *
 * This program and the accompanying materials are dual-licensed under
 * either the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation
 *
 *   or (per the licensee's choosing)
 *
 * under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation.
 */
package ch.qos.logback.core.subst;

import ch.qos.logback.core.CoreConstants;
import ch.qos.logback.core.spi.ScanException;

import java.util.List;

// E = TE|T
//   = T(E|~)
// E = TEopt where Eopt = E|~
// T = LITERAL | { C } |'${' V '}'
// C = E|E :- E
//   = E(':-'E|~)
// V = E|E :- E
//   = E(':-'E|~)

/**
 * Parse a token list returning a node chain.
 *
 * @author Ceki Gulcu
 */
public class Parser {

    final List tokenList;
    int pointer = 0;

    public Parser(List tokenList) {
        this.tokenList = tokenList;
    }

    public Node parse() throws ScanException {
        if (tokenList == null || tokenList.isEmpty())
            return null;
        return E();
    }

    private Node E() throws ScanException {
        Node t = T();
        if (t == null) {
            return null;
        }
        Node eOpt = Eopt();
        if (eOpt != null) {
            t.append(eOpt);
        }
        return t;
    }

    // Eopt = E|~
    private Node Eopt() throws ScanException {
        Token next = peekAtCurentToken();
        if (next == null) {
            return null;
        } else {
            return E();
        }
    }

    // T = LITERAL | '${' V '}'
    private Node T() throws ScanException {
        Token t = peekAtCurentToken();

        switch (t.type) {
        case LITERAL:
            advanceTokenPointer();
            return makeNewLiteralNode(t.payload);
        case CURLY_LEFT:
            advanceTokenPointer();
            Node innerNode = C();
            Token right = peekAtCurentToken();
            expectCurlyRight(right);
            advanceTokenPointer();
            Node curlyLeft = makeNewLiteralNode(CoreConstants.LEFT_ACCOLADE);
            curlyLeft.append(innerNode);
            curlyLeft.append(makeNewLiteralNode(CoreConstants.RIGHT_ACCOLADE));
            return curlyLeft;
        case START:
            advanceTokenPointer();
            Node v = V();
            Token w = peekAtCurentToken();
            expectCurlyRight(w);
            advanceTokenPointer();
            return v;
        default:
            return null;
        }
    }

    private Node makeNewLiteralNode(String s) {
        return new Node(Node.Type.LITERAL, s);
    }

    // V = E(':='E|~)
    private Node V() throws ScanException {
        Node e = E();
        Node variable = new Node(Node.Type.VARIABLE, e);
        Token t = peekAtCurentToken();
        if (isDefaultToken(t)) {
            advanceTokenPointer();
            Node def = E();
            variable.defaultPart = def;
        }
        return variable;
    }

    // C = E(':='E|~)
    private Node C() throws ScanException {
        Node e0 = E();
        Token t = peekAtCurentToken();
        if (isDefaultToken(t)) {
            advanceTokenPointer();
            Node literal = makeNewLiteralNode(CoreConstants.DEFAULT_VALUE_SEPARATOR);
            e0.append(literal);
            Node e1 = E();
            e0.append(e1);
        }
        return e0;
    }

    private boolean isDefaultToken(Token t) {
        return t != null && t.type == Token.Type.DEFAULT;
    }

    void advanceTokenPointer() {
        pointer++;
    }

    void expectNotNull(Token t, String expected) {
        if (t == null) {
            throw new IllegalArgumentException("All tokens consumed but was expecting \"" + expected + "\"");
        }
    }

    void expectCurlyRight(Token t) throws ScanException {
        expectNotNull(t, "}");
        if (t.type != Token.Type.CURLY_RIGHT) {
            throw new ScanException("Expecting }");
        }
    }

    Token peekAtCurentToken() {
        if (pointer < tokenList.size()) {
            return tokenList.get(pointer);
        }
        return null;
    }

}