Exploring Expect

Really Simple Patterns

Occasionally it is useful to prevent the pattern matcher from performing any special interpretation of characters. This can be done using the -ex flag, which causes exact matching. For example, the following command matches only an asterisk.

expect -ex "*"

When using -ex, patterns are always unanchored. The ^ and $ match themselves literally even if they appear as the first or last characters in a pattern. So the following command matches the sequence of characters "^“, "*“, "\n“, and "$“. The usual Tcl interpretations apply. Hence, the \n is still interpreted as a single character.

expect -ex "^*\n$"         ;# matches ^ * \n $

Consider another example:

expect -ex "\\n"

Tcl interprets the \\n as \n and the exact matching occurs with no further backslash processing. This statement matches a backslash followed by an n.

The results of exact matches are written to expect_out(buffer) and expect_out(0,string) as usual, although expect_out(0,string) is necessarily set to the original pattern.

Using -ex may seem like a way to simplify many patterns, but it is really only useful in special circumstances. Most patterns either require wildcards or anchors. And strings such as "foo" are so simple that they mean the same thing when specified via -gl in the first place. However, -ex is useful when patterns are computer-generated (or user-supplied). For example, suppose you are creating Expect patterns dynamically from a program that is printing out SQL lines such as:

select * from tbl.col ...