JavaCC is much easier than it looks.  Search for an example similar to
what you need, and tinker.

http://mindprod.com/jgloss/javacc.html

 (If answering to the following post, one should please not
 quote all of it, but only the short parapgrahs one directly
 refers to.)

 In order to interpret or translate an expression (term), it is
 decomposed into lexical units (tokens, words), which then are
 used by a parser to build symbols and a structured
 representation of the input. This representation then might be
 evaluated or translated into some other representation.

 The syntactial structuring resembles the rules for the
 construction of an expression, which often is given by so-
 called "productions" of the EBNF (extended Backus-Nauer-Form)
 and which sometimes are left-recursive.

 When writing a parser, the left-recursive productions sometimes
 are a worry to the author, because it is not obvious how to
 avoid an infinite recursion. The solution is to rewrite them as
 right-recursive productions.
 
 The addition with a binary infix Operator, for example, is
 left associative. However, it is simpler to analyze in a
 right-associative manner. Therefore, one analyzes the source
 using right-associative rules and then creates a result
 using a left-associative interpretation.
 
 A left-associative grammar might be, for example, as follows.

<numeral> ::= '2' | '4' | '5'.
<expression> ::= <numeral> | <expression> '+' <numeral>.
start symbol: <expression>.

 To analyze this using a recursive descent parser one
 prefers to use the following grammar.

<numeral> ::= '2' | '4' | '5'.
<expression> ::= <numeral>[ '+' <expression> ].
start symbol: <expression>.

 This can be written using iteration as follows.

<numeral> ::= '2' | '4' | '5'.
<expression> ::= <numeral>{ '+' <numeral> }.
start symbol: <expression>.

 However, the product is created in the sense of the
 first grammar. Example code follows.

class Scan
{ static String source = "5+4+2)";
 static int pos = 0;
 static char get(){ return source.charAt( pos++ ); }}
 
class Parse
{
 static int numeral(){ return Scan.get() - '0'; }
 
 static int expression(){ int result = numeral();
   while( '+' == Scan.get() )result += numeral();
   return result; }}
 
public class Start
{
 public static void main( final String[] args )
 { System.out.println( Parse.expression() ); }}

 To be able to parse expressions with higher
 priority, the grammar can be extended.

<numeral> ::= '2' | '4' | '5'.
<product> ::= <numeral> | <product> '*' <numeral>.
<sum> ::= <product> | <sum> '+' <product>.
start symbol: <sum>.

 In iterative notation:
 
<numeral> ::= '2' | '4' | '5'.
<product> ::= <numeral>{ '*' <numeral> }.
<sum> ::= <product>{ '+' <product> }.
start symbol: <sum>.

 In Java:

class Scan
{ static String source = "5+4*2)";
 static int pos = 0;
 static char get( final boolean advance )
 { return source.charAt( advance ? pos++ : pos ); }}
 
class Parse
{
 static int numeral(){ return Scan.get( true ) - '0'; }
 
 static int product(){ int result = numeral();
   while( '*' == Scan.get( false )){ Scan.get( true ); result *= numeral(); }
   return result; }

 static int sum(){ int result = product();
   while( '+' == Scan.get( true ))result += product();
   return result; }}
 
public class Start
{
 public static void main( final String[] args )
 { System.out.println( Parse.sum() ); }}

 Exercises

 - What is the output of the above programs?

 - Extend the last grammar and the last program so as
   to handle subtraction.

 - Extend the result of the last exercise in order
   to handle division.

 - Extend the result of the last exercise so that also
   numbers with multiple digits are accepted.

 - Extend the result of the last exercise so that also
   terms in parentheses are accepted. The input "(2+4)*5)"
   should give the result "30".

 - Extend the result of the last exercise so that
   also a unary minus "-" is recognized.
 
 - Extend the result of the last exercise so that
   more operators and functions are recognized.

 - Extend the result of the last exercise so that
   meaningful error messages are created for all
   inputs that to not fulfill the rules of the input
   language.

 - Extend the result of the last exercise so that the
   error messages also show the location where the error
   was detected. It should be possible to enter an expression
   that spans multiple lines, and an error message should
   contain the number of the line where the error was
   detected.
 
 See also:

JEP - Java Mathematical Expression Parser
http://www.singularsys.com/jep/

Steven Metsker: Building Parsers with Java.
Addison-Wesley 2001, ISBN 0201719622.

A.W. Appel: Modern Compiler Implementation in Java.
Cambridge University Press 1998, ISBN 0521586542.

have you thought of embedding a scripting language into your application? you would get a complete language instead of just some simple expression support. embedding a language should only take a few lines.

OK here my ANTLR gramar:
===
grammar AndOrNotVoter;

@header {
package test;

}

@lexer::header {package test;}

@members {
/** Map variable name to Integer object holding value */
}

orexpression
    :   andexpression ('or' andexpression)*
    ;

andexpression
    : notexpression ('and' notexpression)*
    ;

notexpression
    : ('not')? atom
    ;

atom
    : role
    | LPAREN orexpression RPAREN
    ;

role
    :'ROLE_LEVEL1'
    :'ROLE_LEVEL2'
    :'ROLE_LEVEL3'
    :'ROLE_LEVEL4'
    |'ROLE_EDITOR'
    |'ROLE_OWNER'
    |'ROLE_ADMIN'
    ;

LPAREN    : '('
    ;

RPAREN    : ')'
    ;

WS  :   (   ' '
        |   '\t'
        |   '\r'
        |   '\n'
        )+
        { $channel=HIDDEN; }
    ;
===

I have to evaluate a couple of roles a person can have or may not have
in order to get access to certain data.

So a person that has logged on gets a couple of roles. These roles are
maintained in a class called GrantedAuthorities .
When a certain page is requested I have to check of the granted
Authorities to allow this person access to all or some of the data on
the page.

So there is a custom tag that accepts an expression like;
===example===
(ROLE_OWNER and ROLE_LEVEL4) or ROLE_ADMIN
===/example===
This expression needs to be parsed and compared to the
GrantedAuthorities I get from the logon procedure. I have no idea how to
go on from here... Anyone? I can sure use the help.

It is not like evaluating a mathematical expression where all the data
needed is provided in the expression. In order to give meaning to this
expression I need data from GrantedAuthorities.

How do I go about doing this?

Thanks for your patience

Ivo.

Have you looked at Coco/R?

http://zoogz/~reference/reference/comp_lang.html

If you need a parser for a specific grammar then use the other
advise you have received.

If you just need some form of expression evaluation then
look at the script language integration in Java 1.6 and
JavaScript's nice eval function.

Arne

If strict validation of your expressions is not required (what I think
is your situation), then the above seems not to be very difficult to
implement without any third-party tools.  See the code below (not tested
intensively!).

Regardless of what expression parsing/compiling/processing technique
you'll choose, there is also presented the way I think you could use it
in your code (in this scenario your granted Authorities must simply
implement RoleSet interface).

HTH,
piotr

import java.util.HashSet;
import java.util.Set;
import java.util.StringTokenizer;

public class Roles {

  public interface RoleSet {
    boolean contains(String role);
  }

  public interface CompiledExpression {
    boolean matches(RoleSet roles);
  }

  public static CompiledExpression compile(String expression) {
    return new Compiler(new Tokenizer(expression)).compile();
  }

  static class Tokenizer {
    StringTokenizer st;

    Tokenizer(String expression) {
      st = new StringTokenizer(expression, " ()", true);
    }

    String nextToken() {
      while (st.hasMoreTokens()) {
        String token = st.nextToken().trim();
        if (token.length() > 0) {
          return token;
        }
      }
      return null;
    }
  }

  static class Compiler {
    Tokenizer tokenizer;

    public Compiler(Tokenizer tokenizer) {
      this.tokenizer = tokenizer;
    }

    public CompiledExpression compile() {
      return compile(compile(null));
    }

    CompiledExpression compile(final CompiledExpression left) {
      final String token = tokenizer.nextToken();
      if(token == null) {
        return left;
      }

      abstract class NextExpression implements CompiledExpression  {
        CompiledExpression right;
        NextExpression(CompiledExpression right) {
          this.right = right;
        }
      }

      if(token.equalsIgnoreCase("not")) {
        return compile(new NextExpression(compile(null)) {

          public boolean matches(RoleSet roles) {
            return ! right.matches(roles);
          }
        });
      }
      if(token.equalsIgnoreCase("or")) {
        return new NextExpression(compile(compile(null))) {

          public boolean matches(RoleSet roles) {
            return left.matches(roles) || right.matches(roles);
          }
        };
      }
      if(token.equalsIgnoreCase("and")) {
        return compile(new NextExpression(compile(null)) {

          public boolean matches(RoleSet roles) {
            return left.matches(roles) && right.matches(roles);
          }
        });
      }
      if(token.equals("(")) {
        return compile(new NextExpression(compile(null)) {

          public boolean matches(RoleSet roles) {
            return right.matches(roles);
          }
        });
      }
      if(token.equals(")")) {
        return left;
      }

      return new CompiledExpression() {

        public boolean matches(RoleSet roles) {
          return roles.contains(token);
        }
      };
    }
  }

  public static void main(String[] args) throws Exception {
    final String expression = "a or b and not c";
    final Roles.CompiledExpression ce = Roles.compile(expression);
    System.out.println("expression: " + expression);

    new RoleSet() {
      Set<String> roles = new HashSet<String>();

      public boolean contains(String role) {
        boolean enabled = roles.contains(role);
        System.out.println(
            "role " + role + " " + (enabled ? "set" : "not set"));
        return enabled;
      }

      void check() {
        System.out.println(roles + " " +
            (ce.matches(this) ? "matches" : "don't match"));
      }

      { // test...
        check();

        roles.add("b");
        check();

        roles.add("c");
        check();

        roles.add("a");
        check();
      }
    };
  }
}