Implementation hints

Every RPN expression may be viewed as a sequence of tokens each having both a type and a content. Consider the following expression -1.34 0.34 + sqrt consisting of four tokens: 

Type: Double | Type: Double| Type: Binary plus | Type: square Function
             |             |       operator    |       root
Value: -1.34 | Value: 0.34 | Value: +          | value: sqrt

The following scanner application utilizes pattern matching for decomposing expressions like -1.34 0.34 + sqrt into a token sequence:

Figure 597. Minimalist token scanner Slide presentation 
final String[] patterns = new String[] {
  "sqrt",
  "[-]?([0-9]+[.]?[0-9]*|[.][0-9]+)(E[-]?[0-9]+)?",// Matches e.g. -1.5E-33
  "\\+"};// Escape required avoiding regular expression syntax clash.
final String expression = "2.1 -3.4 sqrt";
try (final Scanner scanner = new Scanner(expression)) {
  while (scanner.hasNext()) {
    for (final String p: patterns) {
      if (scanner.hasNext(p)) {
        System.out.println("Token '" + scanner.next(p) +
            "' matched by '" + p + "'");
        break;
      }
    }
  }
}

The current example expression -1.34 0.34 + sqrt results in:

Figure 598. Scanner output -1.34 0.34 + sqrt Slide presentation 
Token '2.1' matched by '[-]?([0-9]+[.]?[0-9]*|[.][0-9]+)(E[-]?[0-9]+)?'
Token '-3.4' matched by '[-]?([0-9]+[.]?[0-9]*|[.][0-9]+)(E[-]?[0-9]+)?'
Token 'sqrt' matched by 'sqrt'

We provide error handling capabilities dealing with erroneous input ❶:

Figure 599. Adding error detection Slide presentation 
...
final String[] patterns = new String[] {
  "sqrt",
  "[-]?([0-9]+[.]?[0-9]*|[.][0-9]+)(E[-]?[0-9]+)?",// Matches e.g. -1.5E-33
  "\\+"};// Escape required avoiding regular expression syntax clash.
try (final Scanner scanner = new Scanner("2.1 -3.4 pbck") ❶) {
  while (scanner.hasNext()) {
    boolean foundToken = false;
    for (final String p: patterns) {
      if (scanner.hasNext(p)) {
        foundToken = true;
        System.out.println("Token '" + scanner.next(p) +
          "' matched by '" + p + "'");
        break;
      }
    }
    if (!foundToken) {
      System.out.println("Parsing error at '" + scanner.nextLine() + "'");
      System.exit(1);
    } ...

Bogus input like 2.1 -3.4 pbck will now be detected as such:

Figure 600. Error message Slide presentation 
Token '2.1' matched by '[-]?([0-9]+[.]?[0-9]*|[.][0-9]+)(E[-]?[0-9]+)?'
Token '-3.4' matched by '[-]?([0-9]+[.]?[0-9]*|[.][0-9]+)(E[-]?[0-9]+)?'
Parsing error at 'pbck'

A token sequence may then be evaluated using the postfix evaluation algorithm (Read it!).

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