Add :xyz: replacement syntax that removes whatever is before it and inserts whatever is inside it

3 files changed, 41 insertions, 5 deletions

diff --git a/subex/parse.go b/subex/parse.go
index 52488a7..4e7a3f6 100644
--- a/subex/parse.go
+++ b/subex/parse.go
@@ -166,7 +166,7 @@ func parseReplacement(l RuneReader) (output []OutputContent) {
 		switch r {
 			case eof:
 				panic("Missing closing \"")
-			case '=', '^':
+			case '=', '^', ':':
 				break loop
 			case '$':
 				slot := l.Next()
@@ -281,7 +281,7 @@ func parseSubex(l RuneReader, minPower int) SubexAST {
 		case '[':
 			rangeParts := parseRangeSubex(l)
 			lhs = SubexASTRange {rangeParts}
-		case ')', '|', ';', '{', '+', '-', '*', '/', '!', '$':
+		case ')', '|', ';', '{', '+', '-', '*', '/', '!', '$', ':':
 			l.Rewind()
 			return nil
 		case '=':
@@ -358,9 +358,19 @@ func parseSubex(l RuneReader, minPower int) SubexAST {
 				if slot == eof {
 					panic("Missing slot character")
 				}
-				lhs = SubexASTStore{
-					Match: lhs,
-					Slot: slot,
+				if slot == '_' {
+					lhs = SubexASTDiscard {lhs}
+				} else {
+					lhs = SubexASTStore{
+						Match: lhs,
+						Slot: slot,
+					}
+				}
+			case r == ':' && minPower <= 4:
+				replacement := parseReplacement(l)
+				lhs = SubexASTConcat {
+					SubexASTDiscard {lhs},
+					SubexASTOutput {replacement},
 				}
 			case r == '|' && minPower <= 8:
 				rhs := parseSubex(l, 9)
diff --git a/subex/subexast.go b/subex/subexast.go
index ee7a959..dd98aa9 100644
--- a/subex/subexast.go
+++ b/subex/subexast.go
@@ -359,3 +359,16 @@ func (ast SubexASTEmpty) compileWith(next SubexState) SubexState {
 func (ast SubexASTEmpty) String() string {
 	return "()"
 }
+
+// Discards the output from the content subex
+type SubexASTDiscard struct {
+	Content SubexAST
+}
+func (ast SubexASTDiscard) compileWith(next SubexState) SubexState {
+	return &SubexCaptureBeginState {
+		next: ast.Content.compileWith(&SubexDiscardState {next}),
+	}
+}
+func (ast SubexASTDiscard) String() string {
+	return fmt.Sprintf("(%v)$_", ast.Content)
+}
diff --git a/subex/subexstate.go b/subex/subexstate.go
index 997e6ce..b5e1e9b 100644
--- a/subex/subexstate.go
+++ b/subex/subexstate.go
@@ -36,6 +36,19 @@ func (state SubexCaptureBeginState) accepting(store Store, outputStack OutputSta
 	return state.next.accepting(store, outputStack.push(nil))
 }
 
+// Discard the top of the OutputStack
+type SubexDiscardState struct {
+	next SubexState
+}
+func (state SubexDiscardState) eat(store Store, outputStack OutputStack, char walk.Atom) []SubexBranch {
+	_, newStack := outputStack.pop()
+	return state.next.eat(store, newStack, char)
+}
+func (state SubexDiscardState) accepting(store Store, outputStack OutputStack) []OutputStack {
+	_, newStack := outputStack.pop()
+	return state.next.accepting(store, newStack)
+}
+
 // Pop the top of the OutputStack which contains the stuff outputted since the start of the store
 // This outputted data gets stored in a slot
 type SubexStoreEndState struct {