Exploring Expect

More On Anchoring

In the example on page 118, I defined a prompt as “(%|#|\\\$) $”. An obvious question is: “Why do you need those backslashes? (Or perhaps, “Okay, I know I need backslashes, but why three? Why not seventeen or some other random number?!”)

The answer is exactly the same reason as I described in Chapter 4 (p. 91). In this case, the dollar sign is special both to the pattern matcher and to Tcl. This is similar to the problem with the "[“.

To restate, the dollar sign must be prefaced with a backslash to get the pattern matcher to use it as a literal character. Without the backslash, the pattern matcher will use the dollar sign to match the end of the string. However, both the backslash and the dollar sign are also special to Tcl. When Tcl parses command arguments, it will try to make substitutions whenever it sees backslashes and dollar signs.

To avoid losing the backslash, the backslash itself must be prefaced with a backslash. Similarly, the dollar sign must be prefaced with a backslash. The result is "\\" and "\$“, or when combined, "\\\$“.

As always, there is more to this sorry story. The dollar sign substitution made by Tcl only occurs when an alphanumeric character follows. That is, "$foo" is replaced by the value of the variable foo. Since "$" all by itself would imply a variable name of no characters—obviously meaningless—Tcl does not perform any substitution on the dollar sign in such cases. For this reason, you can write the original pattern with two or three ...