Exploring Expect

Chapter 6. Patterns, Actions, And Limits

In this chapter, I will describe the limits of patterns, including what to do when you hit them. I will also cover the darker side of range patterns—matching anything but certain characters. In the second half of the chapter, I will go into more detail on pattern actions including flow control. Finally, I will cover some miscellaneous pattern matching issues that do not fit anywhere else.

Matching Anything But

As I said in Chapter 5 (p. 113), pattern pieces match as many characters as possible. This makes it a little tricky to match a single line, single word, or single anything. For example, the regular expression ".*\n" matches a single line, but it also matches two lines because two lines end with a "\n“. Similarly, it matches three lines, four lines, and so on. If you want to read lines one at a time from another program, then you cannot use this kind of pattern. The solution is to use the "^“.

In Chapter 3 (p. 73), I showed that the "^" matches the beginning of the input buffer. When ^ is the first character of a regular-expression range, it means match anything but the given characters. For example, the regular expression [^ab] matches any character except a or b. The pattern [^a-zA-Z] matches any character but a letter.^[26]

A range can be used to build larger patterns. The pattern "[^ ]*" matches the longest string not including a blank. For example, if the input buffer contained "For example, if the input buffer contained“, the following ...