# Learn regular expression by doing with regetron

When it comes to learning, I buy into the “learning by doing” philosophy. This is especially true when learning a computer tool or programming language. I stumbled upon this regex tutorial that makes use of a set of exercises in conjunction with regetron. Time to hone in on my regex skills.

# Cryptography

This site lists 52 things that people should know about cryptography, if you want to get into the cryptography world. I was also recommended Bruce Schneier’s book, Applied Cryptography: Protocols, Algorthms, and Source Code in C.

# Mahalanobis distance explained

This post does a fantastic job of the Mahalanobis distance. Basically, one can think of it as a multivariate generalization of the z-score. That is, the standardized distance of a vector from the origin (mean vector).

# Break CAPTCHA with cheap labor via services like antigate

I read this interesting post the other day about how to break those annoying CAPTCHA’s by utilizing cheap labor (like \$1 for 1,000 captchas) from less developed countries with services like Antigate. Kind of a cool thought, but that would just mean it’s easier for the spammers to work.

# Handwriting to LaTeX

This site allows me to convert what I write to LaTeX or MathML codes. I think it’s conversion or translation is quite accurate after a quick test.

# Understanding the bin, sbin, usr/bin , usr/sbin split

This post educated me on why there are bin and sbin (superuser) directories in /. /usr/, /usr/local/, and /opt/. The splits came from historical constraints.

# WPS flaw on routers allows WPA protected WIFI networks to be cracked

This post discusses how one could use Reaver to make use of a flaw in WPS to recover a WPA password. Tomato and DD-WRT firmwares don’t support WPS so my network is safe. Lesson: buy and use a router that you can flash Tomato or DD-WRT.

On another note, WEP passwords could also be compromised using BackTrack.

# Annotate or write on top of a pdf file

Sometimes I need to annotate a pdf file, either to take notes or to fill it out as a form since I have ugly handwriting; for the latter, I’m referring to the case where the pdf file does not have form fields you can type in with Adobe Reader. This post describes some programs for annotating pdf files. Xournal is useful when you just want to write on top of a file and print out the content; it converts the content of the original pdf file to an image and saves the old and new contents as a pdf file. I use Xournal on my Asus T101MT touchscreen netbook. Okular seems good except that you can’t save the annotated content and original content as one file or even print the annotated content with the original content.

I just tested Foxit Phantom via Wine and I am able to annotate, save the new pdf file, and print the old+new contents. For form-filling, I recommend printing to pdf as I noticed some glitches with how contents are displayed in the saved pdf files.

# Bond prices and interest rates

I constantly have to review why an inverse relationship exists for bond prices and interest rates. This is a good explanation.

# List open ports on computer using netstat

When you want to know what ports/services are opened on your computer, type the following in the command line:

netstat -tunelp | less