Question

I am creating a JTextField that instantly changes an underscore followed immediately by a number, to a subscript. I need help with the regex code involving replaceAll. I have read a little about regex groups, but I don’t fully understand how to get the number after the underscore in this case.

Subscript code:

// Only 0 - 9 for now...
private String getSubscript(int number)
    {
        String[] sub = {"\u2080", "\u2081","\u2082","\u2083","\u2084","\u2085","\u2086","\u2087","\u2088","\u2089" };
        return sub[number];
    }

Insert Update Code:

public void insertUpdate(DocumentEvent e) {
        if (textField.getText().contains("_"))
        {
            SwingUtilities.invokeLater(this);
        }
    }

Where the actual replace goes (since you can’t directly edit a textfield in the DocumentListener method:

public void run()
    {
        textField.setText(textField.getText().replaceAll("_([0-9])+", getSubscript(Integer.getInteger("$1"))));
    }

This throws a NullPointer exception in the run() method.

Edit:

Here is some example output:

User types “H_2” and immediately this becomes “H₂”, then he continues “H₂O_2” which immediately becomes “H₂O₂”

Answer 1

You can’t do this only using .replaceAll(). You need Pattern and Matcher as follow:

public void run() {

    String text = textField.getText();
    Pattern pattern = Pattern.compile("_[0-9]+");
    Matcher matcher = pattern.matcher(text);

    while (matcher.find()) {
        // get the init string (e.g. "_42")
        String group = matcher.group();
        // parse it as an int (i.e. 42)
        int number = Integer.valueOf(group.substring(1));
        // replace all "_42" with the result of getSubscript(42)
        text = text.replaceAll(group, getSubscript(number));
        // recompile the matcher (less iterations within this while)
        matcher = pattern.matcher(text);
    }

    textField.setText(text);

}