The Old New Thing

May 2006 - Page 2 of 4 - The Old New Thing

How do I write a regular expression that matches an IPv4 dotted address?

May 22, 2006 May 22, 2006 05/22/06
Raymond Chen
Writing a regular expression that matches an IPv4 dotted address is either easy or hard, depending on how good a job you want to do. In fact, to make things easier, let's match only the decimal dotted notation, leaving out the hexadecimal variant, as well as the non-dotted variants. For the purpose of this discussion, I'll restrict myself ...

Redirecting output can result in altered program behavior

May 19, 2006 May 19, 2006 05/19/06
Raymond Chen
Consider a program whose output to the console goes like this. (I've prefixed each line with the output stream.) You want to capture both the normal and error streams, so you run the program and append "" to capture both streams into a single file. But when you look at the resulting output file, you get this: What happened? Most ...

Making up new Winter Olympic events

May 18, 2006 May 18, 2006 05/18/06
Raymond Chen
My approach to inventing new Winter Olympic events is to create new opportunities for head-to-head competition, opening the door to new dramatic possibilities. For example, in Ski Jump Biathlon, one team jumps while the other team tries to shoot them (with paint pellets, of course) as they sail through the air. In Figure Curling, one team ...

The redirection operator can occur in the middle of the command line

May 18, 2006 May 18, 2006 05/18/06
Raymond Chen
Although the redirection operator traditionally appears at the end of a command line, there is no requirement that it do so. All of these commands are equivalent: All of them echo "A B" to the file "C". You can use this trick to avoid the redirection problem we discussed last time. We saw that writing inadvertently interprets the "2...

Beware of digits before the redirection operator

May 17, 2006 May 17, 2006 05/17/06
Raymond Chen
If you want to put the string "Meet at 2" into the file "schedule", you might be tempted to use If you try this, however, you'll see the string "Meet at" on the screen and the "schedule" file will be blank. [Typo fixed, 10am] What happened? A digit immediately before a redirection operator modifies which stream the redirection operator...

The first word on the command line is the program name only by convention

May 15, 2006 May 15, 2006 05/15/06
Raymond Chen
The format of the command line returned by is "", but this is only a convention. If you pass for the to the function, then the function will treat the first word of the as the program name. However, if you pass a value for , then that string determines the program that is run, and the string passed as the is not used for that purpose. ...