BBCode and Regex

BBcode is very popular on forums, if you have never heard of it you may have seen it and don’t know it’s name

BBcode looks like [b][/b] for bold, [url=url]linktitle[/url] for links, etc
I will show you how to make that with Perl and PHP
You need a basic knowledge of regular expressions. I will explain some basic stuff here.

What are Regular Expressions?

Basically, Regular Expressions (also called regexp) are templates that matches a given string.
For example
PHP:

<?php
$string = ‘Hello! Im fedekiller’;
if(preg_match(‘@fedekiller@i’,$string))
{
echo ‘We found fedekiller in that string.’;
}
else
{
echo ‘We could not found fedekiller in that string.’;
}
?>

In Perl:

#!/usr/bin/perl
use CGI’:all’;
print header, start_html();
my $string = ‘Hello! im fedekiller’;
if($string =~ /fedekiller/i)
{
print ‘We found fedekiller in that string.’;
}
else
{
print ‘We could not found fedekiller in that string.’;
}
print end_html;

Those codes will look for fedekiller in the given string, in perl, we use // and put the pattern there, but that was because we did not specify we were matching, if you want to use other separators, you can use for example || or {} but you have to specify you are matching. We will use // here, we dont want to complicate it so much, we also use i after the / to indicate that its CaSe InSeNsItIvE, we also specify case insensitive in PHP after the @
As you have probably realised, to match something in perl we use $string =~/pattern/;
And in PHP we use preg_match(@pattern@,$string);

Quantifiers
Quantifiers are used for repetition
* Matches 0 or more times
+ Matches 1 or more times
? Matches 0 or 1 time
{2} Matches exactly 2 times
{2,} Matches 2 or more
{2,5} Matches 2, 3, 4 or 5 times

Example:
Regex:

fe+dekil{2}er

Will Match:

feeeeeedekiller

Grouping
You can also match by group, using “(” and “)”, that will also make a variable with the content between the “(” and “)”
Example:
Regex:

(fede)+

Will Match:

fedefedefede

We can now acess to the content of the first () with the variable $1, for example if we did something like
Regex:

\[b\](.+?)\[\/b\]

Will Match:

[b]Something[/b]

And the variable $1 will contain Something
Alternatives
You can also match one thing OR other thing, using the ‘|’ bar
Example:
Regex:

(fede|killer)+

Will Match:

killerkillerfedekillerfede

Modifiers:
The modifiers are:
i – Will make it CaSe InSeNsItIvE
g – Global match
m – Multiple lines match
s – Single line match
x – Allow comments and white space in the pattern
e – Evaluate placement
U – Ungreedy pattern
Escape Character
Sometimes we want to use something for example () or {} but we dont want to use that function, for example in BBCode we want to match [b][/b] but no use the []’s function, so, we escape them with the \ character

Anchors
If you want something to match only at the beggining or the end of an string, we can use anchors
^ – Start of a string
$ – End of a string
\b – Word boundary
\B – Not word boundary
\< – Start of a word
\> – End of a word

What is boundary?
Word boundary is if we want to match that string only in a complete word, its for making sure for example we wont find cat in category.

Character Classes
\c – Control Character
\s – White Space
\S – Not white space
\d – Digit (number)
\D – Not digit
\w – Word
\W – Not word
\x – Hexadecimal digit
\O – Octal digit

Special Characters
\n – New Line
\r – Carriage Return
\t – Tab
\v – Vertical tab
\f – Form feed
\xxx – Octal character xxx
\xhh – Hexadecimal character hh
Well, i think that’s enought regex, we can now start with our bbcode function.
So, we want to match all between the [b][/b] tags, and aso the [i], [u], [img] and [url] tags.
In PHP:

function bbcode($content)
{
$content = preg_replace(“@\[b\](.+?)\[\/b\]@i”, “<b>$1</b>”, $content);
$content = preg_replace(“@\[i\](.+?)\[\/i\]@i”, “<i>$1</i>”, $content);
$content = preg_replace(“@\[u\](.+?)\[\/u\]@i”, “<u>$1</u>”, $content);
$content = preg_replace(“@\[img\](.+?)\[\/img\]@i”, “<img src=\”$1\” alt=\”\” />”, $content);
$content = preg_replace(“@\[url\](.+?)\[\/url\]@i”, “<a href=\”$1\”>[Link]</a>”, $content);
$content = preg_replace(“@\[url=(.+?)\](.+?)\[\/url\]@i”, “<a href=\”$1\”>$2</a>”, $content);
$content = str_replace(“\n”, ‘<br />’, $content);
$content = preg_replace(“@\[code\](.+?)\[\/code\]@i”, “<span class=\”box\”>$1</span>”, $content);
return($content);
}

We use the i after the @ to choose case insensitive, we also use the grouping function to get everything between the tags and then a variable wich contains that and put it inside the corresponding HTML tags
We do exactly the same in Perl
In Perl:

sub bbcode
{
$_ = $_[0];
s/\n/<br \/>/gi;
s/\[b\](.+?)\[\/b\]/<b>$1<\/b>/gi;
s/\[i\](.+?)\[\/i\]/<i>$1<\/i>/gi;
s/\[u\](.+?)\[\/u\]/<u>$1<\/u>/gi;
s/\[url\](.+?)\[\/url\]/<a href=”$1″ tager=”_blank”>$1<\/a>/gi;
s/\[url=(.+?)\](.+?)\[\/url\]/<a href=”$1″ target=”_blank”>$2<\/a>/gi;
s/\[img\](.+?)\[\/img\]/<img src=”$1″ \/>/gi;
s/\[code\](.+?)\[\/code\]/<div class=”code”><pre>$1<\/pre><\/div>/gi;
s/\[quote\](.+?)\[\/quote\]/<div class=”quote”>$1<\/div>/gi;
return $_;
}

in PHP, preg_replace is like this preg_replace(regex,replace_with,string)
In Perl
string =~ /regex/replace_with/;
If we dont specify the variable in Perl it will automatically use $_, but we have to specify that we are replacing with the letter s
So it will look like
s/regex/replacement/;

Well, thats all, i hope you all find it useful 🙂