This article describes some useful tools and techniques for learning to build and hone regexps across a range of UNIX® applications, including:

* Highlighting matches
* Showing only the matches and not the lines
* Calling a wizard
* Studying docs

One way to implement content filtering is to use regular expressions in your header, mime_header, nested_header and body checks. This should be simple matches to regular expressions. The goal with this example is to eliminate non-English characters, since we cannot read them anyway.

Every Unix system offers several useful commands for finding files and searching them for strings. Together with programming techniques such as streams, pipes, redirections, and regular expressions they comprise very powerful tools ideal for administrative tasks.

Juliet Kemp shares some useful regular expressions for finding things on your Linux system. She also guides us through the perilous minefields of single quotes, double quotes, and shell expansion, and shows us how to craft our own custom regular expressions.

The term regular expressions refers to a type of search language. You'd use it to search for text strings that match certain defined patterns. So, if you need to search through a document for email addresses, IP addresses, credit card numbers, Social Security numbers, etc., you'd define a regular expression.

When working with regular expressions in a shell script the norm is to use grep or sed or some other external command/program. Since version 3 of bash (released in 2004) there is another option: bash's built-in regular expression comparison operator "=~".

How to use Perl's regular expression lookahead assertion to find overlapping matches.

Swatch isn’t a cleverly designed watch from the 1980s but you’ll think it’s just as handy (and cleverly designed) as one. Like Logwatch, swatch is a perl script that watches your logs but swatch watches them for regular expressions that you configure. Swatch will notify you via mail or the console screen (stdout) when it matches the configured log file entries with your watchfor directives.

There is a classic bit of computer wisdom that states “If you’ve got a problem, and decide to solve it with regular expressions, now you’ve got two problems.” This of course stems from the perception that regular expressions are a complicated mix of magic characters and Voodoo.