java: using regex to parse repeated substrings

Question

This is specifically aimed at parsing hex bytes, but there's a more general question here.

Suppose I have a regexp r e.g. \\s*([0-9A-Fa-f]{2})\\s* (optional spaces, 2 hex digits that I'm interested in, and optional spaces).

If I want to parse a string s with this regexp such that:

• if s can be divided into a sequence of blocks that matches r, I want to do something for each block. (e.g. ff 7c 0903 02BB aC could be divided in this way.)

• If s cannot be divided accordingly, I want to detect this. (e.g. 00 01 02 hi there ab ff and 9 0 2 1 0 and Y0 DEADBEEF and cafe BABE! all fail.)

how could I do this with Java's regexp facilities?

Answer 1

I believe this is a use case for java.util.Scanner. You could use either next(String) or next(Pattern) to discover whether the next token matched your regex.

I don't have a compiler handy, but I think it would go something like this:

Scanner myScanner = new Scanner(mySource);
// default delimiter is any whitespace, so you don't need to call useDelimiter()
Pattern myPattern = Pattern.compile("\\s*([0-9A-Fa-f]{2})\\s*");
String s = null;
while ((s = myScanner.next(myPattern)) != null) {
    // do something with the token
}

Answer 2

Another option would be to use the regex matcher stuff and the lookingAt() method.

Something like:

Pattern p = Pattern.compile( "\\s*([0-9A-Fa-f]{2})" );
Matcher m = p.matcher( myString );
int lastEnd = 0;
while( m.lookingAt() ) {
    System.out.println( "Hex part:" + m.group(1) );
    lastEnd = m.end();
}   
if( lastEnd < myString.length() ) {
    System.err.println( "Encountered non-hex value at index:" + lastEnd );
}

...or whatever. lookingAt() has to start at the current position and so the matches must all be contiguous. The only error condition to catch is finishing early since that means non-hex-formatted data was encountered.

Answer 3

You can check the complete input by adding anchors, or by using matches() instead of contains(), the regexp becomes:

^(\\s*([0-9A-Fa-f]{2}))+\\s*$

If this rgeexp matches, you can then proceed and iterate over the matches for:

\\s*([0-9A-Fa-f]{2})

to pick up the hex bytes.