32.3. Understanding Expressions
2 + 4
"Two plus four" consists of several constants or literal values and an operator. A calculator program must recognize, for instance, that 2 is a numeric constant and that the plus sign represents an operator, not to be interpreted as the + character.
An expression tells the computer how to produce a result. Although it is the sum of "two plus four" that we really want, we don't simply tell the computer to return a six. We instruct the computer to evaluate the expression and return a value.
An expression can be more complicated than 2+4; in fact, it might consist of multiple simple expressions, such as the following:
2 + 3 * 4
A calculator normally evaluates an expression from left to right. However, certain operators have precedence over others: that is, they will be performed first. Thus, the above expression evaluates to 14 and not 20 because multiplication takes precedence over addition. Precedence can be overridden by placing the simple expression in parentheses. Thus, (2+3)*4 or "the sum of two plus three times four" evaluates to 20. The parentheses are symbols that instruct the calculator to change the order in which the expression is evaluated.
A regular expression, by contrast, is descriptive of a pattern or sequence of characters. Concatenation is the basic operation implied in every regular expression. That is, a pattern matches adjacent characters. Look at the following example of a regular expression:
Each literal character is a regular expression that matches only that single character. This expression describes "an A followed by a B followed by an E" or simply the string ABE. The term "string" means each character concatenated to the one preceding it. That a regular expression describes a sequence of characters can't be emphasized enough. (Novice users are inclined to think in higher-level units such as words, and not individual characters.) Regular expressions are case-sensitive; A does not match a.
Programs such as grep (Section 13.2) that accept regular expressions must first evaluate the syntax of the regular expression to produce a pattern. They then read the input, line by line, trying to match the pattern. An input line is a string, and to see if a string matches the pattern, a program compares the first character in the string to the first character of the pattern. If there is a match, it compares the second character in the string to the second character of the pattern. Whenever it fails to make a match, it compares the next character in the string to the first character of the pattern. Figure 32-1 illustrates this process, trying to match the pattern abe on an input line.
Figure 32-1. Interpreting a regular expression
A regular expression is not limited to literal characters. There is, for instance, a metacharacter -- the dot (.) -- that can be used as a "wildcard" to match any single character. You can think of this wildcard as analogous to a blank tile in Scrabble where it means any letter. Thus, we can specify the regular expression A.E, and it will match ACE, ABE, and ALE. It matches any character in the position following A.
The metacharacter * (the asterisk) is used to match zero or more occurrences of the preceding regular expression, which typically is a single character. You may be familiar with * as a shell metacharacter, where it also means "zero or more characters." But that meaning is very different from * in a regular expression. By itself, the metacharacter * does not match anything in a regular expression; it modifies what goes before it. The regular expression .* matches any number of characters. The regular expression A.*E matches any string that matches A.E but it also matches any number of characters between A and E: AIRPLANE, A, FINE, AE, A 34-cent S.A.S.E, or A LONG WAY HOME, for example.
If you understand the difference between . and * in regular expressions, you already know about the two basic types of metacharacters: those that can be evaluated to a single character, and those that modify how characters that precede it are evaluated.
It should also be apparent that by use of metacharacters you can expand or limit the possible matches. You have more control over what is matched and what is not. In Section 32.4 and after, Bruce Barnett explains in detail how to use regular expression metacharacters.
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