Advanced String Processing – How Regular Are Your Expressions

Introduction

Hello hello hello, and welcome
back. We’ve looked at strings, and numbers and all sorts of
types of data, but we’ve not yet seen how to do something
really important, and that’s to look for and pull
interesting parts out of the data we have, to do that where
going to use some magic from the Perl world called “Regular
Expressions”

Huh, Regular What?

Put simply, a regular expression
is a string in it’s own right, but one that has a special
meaning. In some ways it’s like a little mini program that
tells the regexp engine what to look for and how to find it.

Look at the first line of my
opening paragraph above.

If you wanted to look for that
“Hello hello hello” and treat it as a spelling error to
correct how are you going to find it?

Well you could use


if($text == "Hello hello hello")


or you might use the text function “str_replace“:


str_replace("Hello hello hello","Hello");


and they would work fine, but what
if we now made the phrase “Hello hello hullo”? hmmm, it
looks like we now don’t have a match. This is where the
power of regular expressions comes to the rescue.

How do Regular Expressions Work Then?

Ok, so your asking yourself, how
can I match something that’s not match-able unless I change
what I’m looking for, which is not what I want to do.

The key is not to change what your
searching for but, to just search for the differences.

It’s a set of searching rules,
that allows for variations in the text to be searched.

What exactly is the difference
between “Hello hello hello” and “Hello hello hullo” , well
in this case it’s only one letter, and that letter can be
either an ‘e’ or a ‘u’, if we had a way of just saying
search for this phrase, but at the 4th letter from the end,
you need to be aware that that could change, then you’ve
pretty much defined what a regular expression is.

Now in our example here, we could
actually expand that very easily, to be aware of any letter
at that position and not just a ‘e’ or ‘u’, we do this by
using the full stop operator ‘.‘ , so to show
you what I mean we could write our search pattern like this:


"Hello hello h.llo"


That will match any character at
that position, but only one character, all the rest have to
match.

There is however much more to the
power of reg-expressions than just single letters, we can
search for whole groups of numbers, letters and combinations
of words symbols. Also for certain counts and lengths.

There’s far more than we can cover
in this article, developing a true mastery of reg-
expressions takes years. We only have time to cover what you
need to know to get you started in PHP.

Once snippet of advice I will
give, find a program that will allow you to see what your
doing as you construct regular expressions. I use the
wonderful reg-ex coach available from http://weitz.de/regex-
coach/ There is an older version for linux, but the
latest versions are only maintained under Windows now, it
does however run perfectly fine under wine.