Mastering Regular Expressions, 3rd Edition

2Extended Introductory Examples

Remember the doubled-word problem from the first chapter? I said that a full solution could be written in just a few lines in a language like Perl. Such a solution might look like:

$/ = ".\n";while (<>) {  next if !s/\b([a-z]+)((?:\s|<[^>]+>)+)(\1\b)/\e[7m$1\e[m$2\e[7m$3\e[m/ig;  s/^(?:[^\e]*\n)+//mg;   # Remove any unmarked lines.  s/^/$ARGV: /mg;         # Ensure lines begin with filename.  print;}

Yup, that’s the whole program.

Even if you’re familiar with Perl, I don’t expect you to understand it (yet!). Rather, I wanted to show an example beyond what egrep can allow, and to whet your appetite for the real power of regular expressions.

Most of this program’s work revolves around its three regular expressions:

• ⌈\b([a-z]+)((?:\s|<[^>]+>)+)(\1\b)⌋
• ⌈^(?:[^\e]*\n)+⌋
• ⌈^⌋

Though this is a Perl example, these three regular expressions can be used verbatim (or with only a few changes) in many other languages, including PHP, Python, Java, VB.NET, Tcl, and more.

Now, looking at these, that last ⌈^⌋ is certainly recognizable, but the other expressions have items unfamiliar to our egrep-only experience. This is because Perl’s regex flavor is not the same as egrep’s. Some of the notations are different, and Perl (as well as most modern tools) tends to provide a much richer set of metacharacters than egrep. We’ll see many examples throughout this chapter.

About the Examples

This chapter takes a few sample problems — validating user input; working with email ...