Exploring Expect

All Those Other String Functions Are Handy, Too

There are numerous other string manipulation functions that can be used when working with patterns. For example, in Chapter 3 (p. 77), I used "string trimright" to remove all the newline characters from the end of a string.

Another function that is very handy is scan. The scan command interprets strings according to a format. scan is analogous to the C language scanf function. For the most part, scan is less powerful than regexp, but occasionally the built-in capabilities of scan provide exactly the right tool. For example, a regular expression to match a C-style real number is:

([0-9]+.?[0-9]*|[0-9]*.[0-9]+)([eE][-+]?[0-9]+)?

And that is before adding the backslashes in front of "[“, "." and "+“! A much better alternative is to use Tcl’s scan command. This can match real numbers, plus you can constrain it for precision. All you have to do is feed it a string containing a number. You can have expect look for the end of the number (such as by seeing whitespace) and then call:

scan $expect_out(string,0) "%f" num

In this example, the number is stored in the variable num. The %f tells scan to extract a real number. Chapter 2 (p. 46) has more information on scan and other string manipulation commands.