Exploring Expect

Tcl’s regsub Command

The regsub command makes substitutions in a string that matches a regular expression. For example, the following command substitutes like with love in the value of olddiet. The result in stored in the variable newdiet.

expect1.1> set olddiet "I like cheesecake!"
I like cheesecake!
expect1.2> regsub "like" $olddiet "love" newdiet
1
expect1.3> set newdiet
I love cheesecake!

If the expression does not match, no substitution is made and regsub returns 0 instead of 1. However, the string is still copied to the variable named by the last parameter.

Strings that match parenthesized expressions can be referred to inside the substituted string (the third parameter, love in this example). The string that matched the first parenthesized expression is referred to as "\1“, the second as "\2“, and so on up to "\9“. The entire string that matched is referred to as "\0“.

In the following example, cheesecake matches the parenthesized expression. It is first substituted for \1 in the fourth argument, and then that string replaces "cheesecake!" in the original value of olddiet. Notice that the backslash must be preceded by a second backslash in order to avoid Tcl itself from rewriting the string.

expect1.4> set substitute "the feel of \\1 in my nose."
the feel of \1 in my nose
expect1.5> regsub "(c.*e)!" $olddiet $substitute odddiet
1
expect1.6> set odddiet
I like the feel of cheesecake in my nose.

If you find this a little confusing, do not worry. You can usually accomplish the same ...