Secure Programming Techniques

I can’t control how people run my programs or what input they give it, and given the chance, they’ll do everything I don’t expect. This can be a problem when my program tries to pass on that input to other programs. When I let just anyone run my programs, like I do with web applications, I have to be especially careful. Perl comes with features to help me protect myself against that, but they only work if I use them, and use them wisely.

Advanced Regular Expressions

Regular expressions, or just regexes, are at the core of Perl’s text processing, and certainly are one of the features that made Perl so popular. All Perl programmers pass through a stage where they try to program everything as regexes, and when that’s not challenging enough, everything as a single regex. Perl’s regexes have many more features than I can, or want, to present here, so I include those advanced features I find most useful and expect other Perl programmers to know about without referring to perlre, the documentation page for regexes.

Simple word matching

The simplest regex is simply a word, or more generally, a string of characters. A regex consisting of a word matches any string that contains that word:

    "Hello World" =~ /World/;  # matches

In this statement, World is a regex and the // enclosing /World/ tells perl to search a string for a match. The operator =~ associates the string with the regex match and produces a true value if the regex matched, or false if the regex did not match. In our case, World matches the second word in "Hello World", so the expression is true. This idea has several variations.
Expressions like this are useful in conditionals:

A Beginner's Introduction to Perl 5.10, part 2

The first two articles in this series (A Beginner's Introduction to Perl 5.10 and A Beginner's Introduction to Files and Strings in Perl 5.10) covered flow control, math and string operations, and files. (A Beginner's Introduction to Perl Web Programming demonstrates how to write secure web programs.) Now it's time to look at Perl's most powerful and interesting way of playing with strings, regular expressions, or regexes for short. The rule is this: after the 50th time you type "regular expression", you find you type "regexp" ever after.
Regular expressions are complex enough that you could write a whole book on them (Mastering Regular Expressions by Jeffrey Friedl).

Beginner's Introduction to Perl 5.10

By chromatic on May 7, 2008 12:00 AM

A Beginner's Introduction to Perl 5.10 talked about the core elements of Perl: variables (scalars, arrays, and hashes), math operators and some basic flow control (the for statement). Now it's time to interact with the world. (A Beginner's Introduction to Regular Expressions with Perl 5.10 explores regular expressions, matching, and substitutions. A Beginner's Introduction to Perl Web Programming demonstrates how to write web programs.)
This installment discusses how to slice and dice strings, how to play with files and how to define your own functions. First, you need to understand one more core concept of the Perl language: conditions and comparisons.