Perl Basics

Perl Basics has two main goals. First, it provides a succinct summary of major Perl elements. Second, it provides perspective and relates features to one another. Thus, you may think of it as an extended and structured checklist, with commentary.
The discussion is oriented toward answering two questions:

• What are the things Perl provides you to work with?
• What can you do to those things?

The discussion includes six major sections:

1. Variables and their Related Operators
2. Control structures
3. Functions
4. Regular Expressions
5. Input/Output
6. System Operators

---

1. Variables and their Related Operators

Perl provides three kinds of variables: scalars, arrays, and associative arrays. The discussion includes the designation of these three types and the basic operators provided by Perl for their manipulation.

2. Control Structures

Perl is an iterative language in which control flows from the first statement in the program to the last statement unless something interrupts. Some of the things that can interrupt this linear flow are conditional branches and loop structures. Perl offers approximately a dozen such constructs. Each of these basic constructs are described along with examples illustrating their use.

3. Functions

Functions are a fundamental part of most programming languages. They often behave like an operator, producing a change in the value of some variable or returning a value that can be assigned to a variable. They also control the flow of execution, transferring control from the point of invocation to the function definition block and back. Thus, they combine properties of the two preceding discussions. The discussion will cover both the designation of functions and their invocation and use.

4. Regular Expressions and Related Operators

Regular expressions are strings that can be recognized by a regular grammar, a restricted type of context-free grammar. Basically, they are strings that can be parsed left to right, without backtracking, and requiring only exact symbol matching, matching of a symbol by a category of symbols, or matching of a symbol by a specified number of sequential occurrences of a symbol or category. Perl provides a general mechanism for specifying regular expressions. It also provides several operators that manipulate strings based upon the evaluation of a regular expression.
The discussion will begin by describing the various mechanism for specifying patterns and then discuss expression-based operators.

5. Input/Output

Perl provides basic I/O for both the standard input (keyboard) and output (display) devices and for files in the UNIX file system. More sophisticated I/O is provided through the UNIX DBM library. These various I/O capabilities are discussed.

6. System Operators

Perl offers a number of operators that mimic or call UNIX system operators or analogous operators for other operating systems. The discussion here will be cast in the context of UNIX and will assume familiarity with basic UNIX facilities. Perl system operators can be roughly divided into two large categories: file/directory operators and process operators. Both types are discussed.