Testing the PCRE regex

The following tools can be used to test the PCRE regexes (Perl Compatible Regular Expressions) in the format file. Bear in mind that some characters, including the "\" (backslash) character, have to be escaped in the strings in the json format file, so two backslashes ("\\") should be written there instead of one.

1. The https://regex101.com/ website is really useful for testing and writing regular expressions. Copy the work-in-progress regex there, and a line to be matched from xensource.log, and modify the regex until it matches.

2. Then test with the pcregrep tool, which is a grep tool that uses PCRE regexes, that the regex matches every line in a xensource.log file. By inverting the match, the pcregrep -v <regex to be tested> command will output all the non-matching lines.

   For the emitted non-matching lines, go back to step 1. and amend the regex until it matches the line, then repeat the procedure.