Confusion with ereg_replace() Beginning PHP and MySQL Example by W Jason Gilmore

Question

Just a note to begin I am aware that ereg_replace() is deprecated, since POSIX is no longer being used. But in "Beginning PHP and MySQL" by W Jason Gilmore, Gilmore emphasizes that although POSIX isn't to be used, an understanding is still necessary as a means of conversion to Perl. So once again I understand it's deprecated but since I'm trying to understand everything in the book I might as well understand this.

So the example is as follows:

<?php
   $text = "This is a link to http://www.example.com/.";
   echo ereg_replace("http://([a-zA-Z0-9./-]+)$", "<a href=\"\\0\">\\0</a>", 
                     $text);
?>
//Output
This is a link to <a href="http://www.example.com/">http://www.example.com/.</a>.

So I understand the majority of code in the above example, my problem lies with the ./- and the output. For the ./- I tried to think according to quantifiers where . = between, so everything between [:alnum:] and / is replaced. I also thought maybe ./- are characters within the range which would also be replaced since [:alnum:] doesn't include punctuation. For verfication I looked at the output but theres no - present. If only the / is replaced than the code would make sense, since /0 outputs http://www.example.com/ but than the problem lies with the missing - which I presume to be pertinent to the brackets rather than as a quantifier.

My other question is in regards to the output, if the function returns the string with the modified string why does the period which was present in the original string appear after the second /0, not the first, if its the original text, why does the tag follow it and not precede it?

Just for some quick background, I have a basic understanding of php,html,css,javascript,C++ and I'm reading this for a more in depth understanding of php and an introduction to MySQL, so unfortunately explanations which are entirely advanced code/concepts go right over my head.

Answer 1

why does the period which was present in the original string appear after the second /0, not the first

This is not the case, because the actual output is:

This is a link to <a href="http://www.example.com/.">http://www.example.com/.</a>

The period is included in both the attribute as well as the tag contents.

my problem lies with the ./- and the output

When present inside a character set, ./- means to match either a period, forward slash or a dash; it's important to note that the dash must appear at the end of the character set to avoid ambiguity.