6.5. The Format of the OPTIONS clause The format of the OPTIONS clause is very simple, it is followed by any REXX string expression, which is interpreted as a set of space separated words. The words are treated strictly in order from left to right, and each word can change zero or more extension settings. Each extension has a name. If the word being treated matches that name, that extension will be turned on. However, if the word being treated matches the name of an extension but has the prefix NO, then that extension is turned off. If the word does not match any extensions, then it is simply ignored, without creating any errors or raising any conditions. Example: Extensions changing parsing An example of the same is the UPPER instruction. In the following piece of code the same clause is interpreted in two completely different ways: options ‘NOUPPER’ do i=1 to 2 if i=2 then options ‘UPPER’ upper foo bar end In the first iteration of the loop, the clause starting with the token UPPER will be a command, issuing the string resulting from evaluating the expression upper foo bar. However, in the second iteration of the loop, the same clause is interpreted as an UPPER instruction. Since these two statements has very different syntax, it seems impossible to handle both in the same program. Regina tries to handle this by “allowing” both syntaxes when parsing the source code, and selecting the right one when interpreting the statement in question. Regina’s frequent usage of extensions may slow down execution. To illustrate how this can happen, consider the OPEN() extra built-in function. As this is an extension, it might be dynamically included and excluded from the scope of currently defined function. Thus, if the function is used in a loop, it might be in the scope during the first iteration, but not the second. Thus, Regina can not cache anything relating to this function, since the cached information may be outdated later. As a consequence, Regina must look up the function in the table of functions for each invocation. To avoid this, you can set the extension CACHEEXT, which tells Regina to cache info whenever possible, without regards to whether this may render useless later executions of OPTIONS.