I wanted to write a VI that would take an array of strings, a Search Pattern that adheres to the "simple Regular Expression" rules used by the Match Pattern function and the Search and Replace (with Regular Expression enabled), and return the sub-array matching the pattern.
I started with Search and Replace, with Regular Expressions turned on. I set "Ignore Case" to True, and didn't wire the Replace String. I used as a "Match occurred" the expression "Number of Replacements > 0" (I get the same result if I use "Offset past Replacement < 0"). When my Search String is an empty string, it returns 0 replacements, and Offset past Replacement of -1, though the Detailed Help says "If search string is an empty string and replace all? is FALSE, the function inserts replace string at the beginning of input string." I (naively?) interpret this as "If the search string is an empty string and replace all? is False, there will be one replacement made at the beginning of the input string". I also assume (but this could also be wrong!) that replacing something with an empty string is still a replacement.
I tried doing the same thing with the Match Pattern function, which does not have a "Replace all?" option and does not return the Number of Replacements. It (correctly, in my opinion) tells me when my Search string is an empty string, it inserted the (empty) Replacement string at Position 0 of my Input string, i.e. it behaves the way the Help description suggests that it should (and has the functionality I need).
Two questions. First, is this a "Bug" or a "Feature" of Search and Replace (with Regular Expressions)? It certainly seems like a "Bug" in not adhering to the Help Documentation, but it has probably been working this way as a Feature for too long to risk "fixing" and breaking who-knows-how-many existing VIs. An alternative to Fixing the Bug is to Fix the Documentation to clarify this behavior.
Second question is what is the rationale for having slightly different behaviors for these two functions? True, they are slightly different (Match Pattern has fewer options and no Error Lines), but it's not really clear (other than Legacy Support) why both are needed. It is even less clear why they have different behaviors when the Search string is an empty string (certainly wasn't clear to me from the documentation).
Bob (Trouble-Maker) Schor