Understanding Regular Expressions - part 1

Using regular expressions provides us with enormous power, but reading and understanding complex regular expressions is not an easy task. I noticed, it feels much easier to write a regular expression than to read it.

This is probably true regardless of the host language but in Perl we tend to use regexes much more than in other languages.

IMHO the ease of understanding regexes is just a question of practice.

If you practice it enough, you will stop seeing them as a set of random characters.

So in a number of writings I'll take regex examples from open source projects and I'll try to understand and explain them. I don't have a clear path yet. I hope after seeing and writing about a few examples I'll have a better understanding how to explain the regexes and the process of understanding them in a coherent way.

Some of the regexes will be simple ones. I am still practicing the explanation too.

If you have a good regex example or one that you don't understand, please send it to me with a link to its source. I'll see if I can understand it and explain it.

Command line parameters

Looking at Getopt::Std it has a line like this:

    while (@ARGV && ($_ = $ARGV[0]) =~ /^-(.)(.*)/) {

The regex /^-(.)(.*)/ is quite simple. ^ at the beginning of the string there must be a dash -. After that any character matched by . and captured by the parentheses (.) into $1. The (.*) will capture any leftover characters from the string.

So this regex will match things like "-a", "-abc", but it won't match "--xyz" nor "abc".

This is of course not surprising as the purpose of Getopt::Std is to handle command line parameters with single leading dash.

Look-ahead assertion

Still in Getopt::Std I saw these two lines:

	my (@witharg) = ($args =~ /(\S)\s*:/g);
	my (@rest) = ($args =~ /([^\s:])(?!\s*:)/g);

The first one is very simple. (\S) captures a single non-white space character then \s* will eat all white spaces (but does not require any) and then a colon : is required. The whole thing is matched globally (because of the /g at the end and will return the non-whitespace characters matched by (\S).

So if $args = 'a: b : cde: +: -', the @witharg will be ('a', 'b', 'e', '+').

Those are the flags that need to come with a value.

I am not sure why does this code accept any non-word character as flag but maybe that's just my conservative view.

The second regex is a bit more complex. It has two groups. The first one ([^\s:]) capture a (single) character that is NOT white-space and NOT colon. (so it can be a letter or another special character. The second group (?!\s*:) is a zero-width negative look-ahead assertion. Disregard the ?! and you will see it tries to match 0 or more white spaces followed by a colon. The leading ?! means that this is a zero-width negative look-ahead which means there is either no colon after the first group matched or if there is a colon it is separated by at least one non-space character from the match of the first group.

So if we try to match with the same string 'a: b : cde: +: -' we'll get the list of the other non-whitespace and non-colon character: ('c', 'd', '-')

Alternate

  $has_pod = 1, last if /^=(pod|head1)/;

This code is easy. ^ - at the beginning there is an equal sign = followed by either the word "pod" or the word "head1".

It is not checked if the string is =podcast or =head123. Both of these will be accepted.

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