This Java applet demonstrates the 'contains' method of the jrexx Pattern class, which checks if a given text argument matches the pattern's regular expression.

Instructions

1. Insert a regular expression (see regex syntax) into the combobox.
2. Insert an input text into the left text area
3. Click the 'Check input' button
4. The result will be shown in the right text area

The second Java applet demonstrates the use of the internal automaton class for a more sophisticated task. This applet checks the input text on every text change event (e.g. key press) and optionally blocks the change, if the resulting text doesn't match the regular expression. A detailed description can be found below.

Instructions

1. Click on the 'define' button and define a regular expression (see regex syntax).
2. Insert an input text into the textarea
3. Optionally toggle the 'block invalid keys' checkbox
4. See how the textarea's background switches the color to indicated if the input text is valid (white) or invalid (pink)

Description
The demo above has two diffferent features which are implemented separately.

1. Changing colors
   This is easy to achive. We just have to call the Pattern's contains() method everytime after the input text has changed. If the result is true, which means that the input text matches the regular expression, we set the background of the textarea to white, otherwise to pink.
2. Blocking keys
   When you play with the demo, you'll see, that you normally can't insert an invalid text while the checkbox is checked. But you are still able to delete any characters from the text even although the resulting text might become invalid (which is really a feature). Also you might notice, that the algorithm just checks the input text between index 0 and the cursor position.
   
   To achive this, the following method is called on every text insertion and returns the number of characters from the given text, that can be processed by the pattern's automaton. Please note, that this does not mean, that these characters form a valid String that matches the complete regular expression. Even if the whole text's length is returned by the method, the text still might be invalid, because the regular expression might still miss characters at the end of the text. If you want, you can say, that the text's substring (from index 0 to the method's resulting int) still has a chance to become a valid string (in the sense of the regex), just by appending valid characters.

   protected int getMatchLength( String text) { if ( this.pattern == null) return 0; final int textLength = text.length(); IState state = this.pattern.getAutomaton().getStartState().visit(); for (int i=0; i<textLength; ++i) { state = state.next( text.charAt(i)); if (state==null) return i; } return textLength; }

Since February 12th we also have jrexx-Lab online. This is a graphical editor for automaton based regular expressions build upon the jrexx api.

Don't miss the jrexx-Lab Java Webstart page.