• Perl 101 - October 4
• Perl 201 - October 11

Sign up early for the early bird discount ($50 off) and take an extra 10% off if you enroll in more than class at once, or enroll as a group with one or more other people (limit one 10% discount per person). Email enroll@bayview.com today for more information.

If these dates don’t work for you, please feel free to contact us to discuss alternate dates, or for one-on-one coaching or on-site training at your location. We can be very flexible with scheduling to accommodate your needs.

• Shell 101 - Using the Unix/Linux command line, including Bourne shell, tcsh, and text-processing utilities
• Shell 201 - Writing shell scripts in Bourne Again Shell (bash)
• Regex 101 - All about regular expressions, including grep, text editing (vi or emacs), Perl, and more

If you have any comments, questions, or suggestions please let us know by email at training@bayview.com.

Perl 101 - September 22
Perl 201 - October 6 and 20 (two half-day classes)

Sign up early for the early bird discount ($50 off all-day classes or $30 off half-day classes) and take an extra 10% off if you enroll in more than class at once, or enroll as a group with one or more other people (limit one 10% discount). Email enroll@bayview.com today for more information.

If these dates don’t work for you, please feel free to contact us to discuss alternate dates, or for one-on-one coaching or on-site training at your location. We can be very flexible with scheduling to accommodate your needs.

• Perl 101 - April 19
• Perl 201 - April 26

Sign up early for the early bird discount ($50 off) and take an extra 10% off if you enroll in more than class at once, or enroll as a group with one or more other people (limit one 10% discount per person). Email enroll@bayview.com today for more information.

If these dates don’t work for you, please feel free to contact us to discuss alternate dates, or for one-on-one coaching or on-site training at your location. We can be very flexible with scheduling to accommodate your needs.

Say, for example, that you need to change permissions on a file. On Unix and related operating systems (including Mac OS X and Linux) you would use the chmod function, but that doesn’t do much on Windows. Although the chmod command will execute on Windows it doesn’t do much. Traditional FAT filesystems have only four attributes per file: archived (A), read-only (R), hidden (H), and system (S). These can be checked and set with the Win32::File module. For NTFS, use Win32::FileSecurity.
(For more information about Perl on Windows, see the Perl Win32 FAQ.)

So, in your program, if you want to be able to do one thing on Windows and another on Unix-like systems, you need to test what platform you are on. The way to do that in Perl is to look at the special variable $^O. (That’s the letter O, not the number 0). If you’re on Windows, it will have the value “MSWin32″ (sadly, it doesn’t give you any clue what version of Windows you’re on). So you would do something like this:

if ($^O eq "MSWin32") {
    require Win32::FileSecurity;
    Win32::FileSecurity::Set($file, { ... })
        or die "Error in FileSecurity for $file: $^E";
}
else {
    chmod 0644, $file
        or die "Error in chmod for $file: $!";
}

To find out what the value of $^O is on your platform, run this little command from your shell:

perl -e "print $^O";

my($name) = /hello, (\w+)/

In this example, we look in $_ for the word “hello” followed by a comma, space, and a word. Since the word, \w+, has parentheses around it, the part of the string that it matches gets memorized. In this example, we are assigning the return value of the regular expression match to $name. So if $_ contains “hello, world” then $name gets “world” - very convenient.

But parentheses also do other things besides memorize their contents, and this feature can become annoying. Here’s an example.

In a regular expression the | symbol indicates “or” - either the stuff to the left of it or the stuff to the right of it will match. For example, /hello|hi/ will match either “hello” or “hi” in the string. You can even have more than one of these: /hello|hi|howdy|greetings/ will match any of those four words.

The trouble is, what if you want the “or” to apply to only part of the string? That’s where parentheses come in. Let’s combine the previous two examples to show what I mean:

my($name) = /(hello|hi|howdy|greetings), (\w+)/

In this example, we want any of “hello,” “hi,” “howdy,” or “greetings”, followed by a comma, space, and a word which is memorized. The problem is, the greeting word is also memorized, and so $name gets that word instead of the name that we want it to get!

The easy solution is to allocate a variable for that word:

my($x, $name) = /(hello|hi|howdy|greetings), (\w+)/

But here, we don’t care about the value in $x so why bother allocating a variable for it? Can’t get just this one benefit of parentheses without having them memorize anything? For years, the answer was no. But then a few years back the Perl regex guys came up with a syntax to do it - just add ?: to the beginning of the parenthesized block, making it:

my($name) = /(?:hello|hi|howdy|greetings), (\w+)/

Gee, that was awfully obvious, wasn’t it? NOT! Why do they have to make these things so unintelligible? I hear you cry.

The answer is backward compatibility. Think about it - all the obvious characters already mean something, or if they don’t, chances are someone’s used them in a regular expression already to search for that character. So the only way to introduce a new feature into regular expressions is to use something that previously was a syntax error. Since the “?” character in a regex means “the previous thing zero or one times” and the thing before the “?” in this syntax is “(” (which if you recall means “start memorizing here”), it didn’t make sense to say “start memorizing here, zero or one times” so it was a syntax error. Since it was an error, nobody would have used it in an existing Perl script. So by giving (? a meaning that wasn’t a syntax error, backward compatibility is preserved.

But why “(?:” and not just “(?“? I wasn’t there, but I would assume they wanted to add more features to the parenthesized syntax and were running out of previously-bad syntax that they could give meaning to. For example, you may know that you can make a regex case-insensitive by adding /i to the end. Well, you can also insert the “i” between the “?” and “:” to make only part of the regex be case-insensitive: /(?i:hello|hi), world/ would allow “hello” or “HELLO” but “world” would have to be all lowercase.

So, the bottom line: if you find yourself wanting to use parentheses in your regex for reasons other than memorizing, and memorizing gets in your way (or you want to save a little on performance, since memorizing can slow things down a little), then just remember to insert ?: at the start of the parenthesized part of your pattern.

my($name) = /(?:hello|hi|howdy|greetings), (\w+)/

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