foreach command

Something like this:

set example {custom_string_foo=[get this data w/out the brackets], custom_string_bar=[get this data too],custom_string_again=[keep getting data until no more instances of custom_string]}

foreach x [split $example ","] {
    log local0. [findstr $x "\[" 1 "\]"]
}

The split command breaks the larger string into list items, and then the foreach loops through that list and extracts the string between the brackets using the findstr command.