1 files changed, 27 insertions, 0 deletions
diff --git a/main/lex.go b/main/lex.go
index 0daf2d1..e82c309 100644
--- a/main/lex.go
+++ b/main/lex.go
@@ -147,6 +147,7 @@ const (
 	TokenNumberLiteral // A number literal
 	TokenPatternStringIndex // A string index in a pattern
 	TokenPatternIntegerIndex // An integer index in a pattern
+	TokenSubex // A subex
 )
 
 type Token struct {
@@ -269,6 +270,9 @@ func lexCommand(l *lexer) stateFunc {
 		case 'i':
 			l.emit(TokenCommand)
 			return lexMultipleLiterals
+		case 's':
+			l.emit(TokenCommand)
+			return lexSubstitution
 		case 'S':
 			l.emit(TokenCommand)
 			return lexBigSubstitution
@@ -280,6 +284,29 @@ func lexCommand(l *lexer) stateFunc {
 	return l.errorf("Expected command found something else")
 }
 
+func lexSubstitution(l *lexer) stateFunc {
+	delimiter := l.next()
+	if delimiter == eof {
+		return l.errorf("Missing subex in substitution command")
+	}
+	l.emit(TokenSubstituteDelimiter)
+	loop: for {
+		r := l.next()
+		switch r {
+			case delimiter:
+				l.backup()
+				l.emit(TokenSubex)
+				l.next()
+				l.emit(TokenSubstituteDelimiter)
+				break loop
+			case eof:
+				return l.errorf("Missing closing substitution delimiter")
+			default:
+		}
+	}
+	return lexCommand
+}
+
 func lexBigSubstitution(l *lexer) stateFunc {
 	delimiter := l.next()
 	if delimiter == eof || isAlphaNumeric(delimiter) {