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e425. Applying Regular Expressions on the Contents of a File

The matching routines in java.util.regex require that the input be a CharSequence object. This example implements a method that efficiently returns the contents of a file in a CharSequence object.

    // Converts the contents of a file into a CharSequence
    // suitable for use by the regex package.
    public CharSequence fromFile(String filename) throws IOException {
        FileInputStream fis = new FileInputStream(filename);
        FileChannel fc = fis.getChannel();
    
        // Create a read-only CharBuffer on the file
        ByteBuffer bbuf = fc.map(FileChannel.MapMode.READ_ONLY, 0, (int)fc.size());
        CharBuffer cbuf = Charset.forName("8859_1").newDecoder().decode(bbuf);
        return cbuf;
    }

Here is sample code that uses the method:

    try {
        // Create matcher on file
        Pattern pattern = Pattern.compile("pattern");
        Matcher matcher = pattern.matcher(fromFile("infile.txt"));
    
        // Find all matches
        while (matcher.find()) {
            // Get the matching string
            String match = matcher.group();
        }
    } catch (IOException e) {
    }