Makes regular expression selection on collections key case insensitive

This issue was initially reported by @michaelgranzow-avi on #2296.

@airween made an initial attempt to provide a fixed at #2107; As a
consequence of the pull request review - provided by @victorhora,
@zimmerle, and @michaelgranzow-avi - @airween made a second attempt
at #2297. After reviewing by @martinhsv, @zimmerle, I have absorbed
the essential pieces from @airween patch into this one.

This patch differs from @airween's because @airween's patches were
partially working: Key exclusions with regex weren't covered, same
for anchored variables (e.g. ARGS). During the review, I have
highlighted the importance of having elementary test cases. A simple
test case on ARGS could spot the issue. Since that is an important
fix, I don't want to hold this for one more review cycle; therefore,
I am committing the fix myself.

Thank you all involved in the solution of this very own issue.

src/utils/regex.cc

@@ -52,12 +52,16 @@ bool crlfIsNewline() {
     return crlf_is_newline;
 }
 
-Regex::Regex(const std::string& pattern_)
+Regex::Regex(const std::string& pattern_, bool ignoreCase)
     : pattern(pattern_.empty() ? ".*" : pattern_) {
     const char *errptr = NULL;
     int erroffset;
+    int flags = (PCRE_DOTALL|PCRE_MULTILINE);
 
-    m_pc = pcre_compile(pattern.c_str(), PCRE_DOTALL|PCRE_MULTILINE,
+    if (ignoreCase == true) {
+        flags |= PCRE_CASELESS;
+    }
+    m_pc = pcre_compile(pattern.c_str(), flags,
         &errptr, &erroffset, NULL);
 
     m_pce = pcre_study(m_pc, pcre_study_opt, &errptr);

src/utils/regex.h

@@ -61,7 +61,7 @@ struct SMatchCapture {
 
 class Regex {
  public:
-    explicit Regex(const std::string& pattern_);
+    explicit Regex(const std::string& pattern_, bool ignoreCase = false);
     ~Regex();
 
     // m_pc and m_pce can't be easily copied