There are certainly ways of filtering out all possibly evil HTML and Javascript, but they may be too restrictive for your application. For instance, you could launder CGI data through /([a-zA-Z0-9_&;\s]*)/, which would disallow all HTML except for entities, but this would be much too restrictive for a site like PerlMonks where we need to be able to post code.

Constructing a character class that filters out bad stuff is trivial. On the other hand, constructing a hack-proof set of regexen that permit specific combinations of characters while disallowing others (as in allow <a> but disallow <script> while allowing '<' and '>' if inside a code block) is far from easy.

Everything's implementation of the latter is something you might want to take a look at.

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.