song/StringFilter: support regular expressions with "=~" and "!~"

This feature requires `libpcre`.

src/song/Filter.cxx

@@ -207,6 +207,20 @@ static StringFilter
 ParseStringFilter(const char *&s, bool fold_case)
 {
 	bool negated = false;
+
+#ifdef HAVE_PCRE
+	if ((s[0] == '!' || s[0] == '=') && s[1] == '~') {
+		negated = s[0] == '!';
+		s = StripLeft(s + 2);
+		auto value = ExpectQuoted(s);
+		StringFilter f(std::move(value), fold_case, false, negated);
+		f.SetRegex(std::make_shared<UniqueRegex>(f.GetValue().c_str(),
+							 false, false,
+							 fold_case));
+		return f;
+	}
+#endif
+
 	if (s[0] == '!' && s[1] == '=')
 		negated = true;
 	else if (s[0] != '=' || s[1] != '=')

src/song/StringFilter.cxx

@@ -31,6 +31,11 @@ StringFilter::MatchWithoutNegation(const char *s) const noexcept
 	assert(s != nullptr);
 #endif
 
+#ifdef HAVE_PCRE
+	if (regex)
+		return regex->Match(s);
+#endif
+
 	if (fold_case) {
 		return substring
 			? fold_case.IsIn(s)

src/song/StringFilter.hxx

@@ -23,7 +23,12 @@
 #include "lib/icu/Compare.hxx"
 #include "util/Compiler.h"
 
+#ifdef HAVE_PCRE
+#include "lib/pcre/UniqueRegex.hxx"
+#endif
+
 #include <string>
+#include <memory>
 
 class StringFilter {
 	std::string value;
@@ -33,6 +38,10 @@ class StringFilter {
 	 */
 	IcuCompare fold_case;
 
+#ifdef HAVE_PCRE
+	std::shared_ptr<UniqueRegex> regex;
+#endif
+
 	/**
 	 * Search for substrings instead of matching the whole string?
 	 */
@@ -53,6 +62,21 @@ public:
 		return value.empty();
 	}
 
+	bool IsRegex() const noexcept {
+#ifdef HAVE_PCRE
+		return !!regex;
+#else
+		return false;
+#endif
+	}
+
+#ifdef HAVE_PCRE
+	template<typename R>
+	void SetRegex(R &&_regex) noexcept {
+		regex = std::forward<R>(_regex);
+	}
+#endif
+
 	const auto &GetValue() const noexcept {
 		return value;
 	}
@@ -70,7 +94,9 @@ public:
 	}
 
 	const char *GetOperator() const noexcept {
-		return negated ? "!=" : "==";
+		return IsRegex()
+			? (negated ? "!~" : "=~")
+			: (negated ? "!=" : "==");
 	}
 
 	gcc_pure

src/song/meson.build

@@ -19,6 +19,7 @@ song_dep = declare_dependency(
   link_with: song,
   dependencies: [
     icu_dep,
+    pcre_dep,
     tag_dep,
     util_dep,
   ],