Added support for start-of-input (\A) and end-of-input (\Z) assertions

regex/nfa.go

@@ -11,8 +11,10 @@ type assertType int
 
 const (
 	noneAssert assertType = iota
-	sosAssert
-	eosAssert
+	sosAssert             // Start of string (^)
+	soiAssert             // Start of input (\A)
+	eosAssert             // End of string ($)
+	eoiAssert             // End of input (\Z)
 	wboundAssert
 	nonwboundAssert
 	plaAssert        // Positive lookahead
@@ -119,6 +121,15 @@ func (s nfaState) checkAssertion(str []rune, idx int) bool {
 		// Index is at the end of the string, or it points to the last character which is a newline
 		return idx == len(str) || (multilineMode && str[idx] == '\n')
 	}
+	if s.assert == soiAssert {
+		// Only true at the start of the input, regardless of mode
+		return idx == 0
+	}
+	if s.assert == eoiAssert {
+		// Only true at the end of the input, regardless of mode
+		return idx == len(str)
+	}
+
 	if s.assert == wboundAssert {
 		return isWordBoundary(str, idx)
 	}