2 files changed, 7 insertions, 3 deletions
diff --git a/src/interpreter/PeggyParser.js b/src/interpreter/PeggyParser.js
index 121cf9d..f5376b3 100644
--- a/src/interpreter/PeggyParser.js
+++ b/src/interpreter/PeggyParser.js
@@ -215,7 +215,7 @@ export default (function () {
     var peg$c2 = ".";
     var peg$c3 = "\r\n";
 
-    var peg$r0 = /^[a-zA-Z0-9]/;
+    var peg$r0 = options.allowUnderscores ? /^[a-zA-Z0-9_]/ : /^[a-zA-Z0-9]/;
     var peg$r1 = /^[\\\u03BB]/;
     var peg$r2 = /^[\t-\n ]/;
 
diff --git a/src/interpreter/parser.ts b/src/interpreter/parser.ts
index 5e3be0f..930b953 100644
--- a/src/interpreter/parser.ts
+++ b/src/interpreter/parser.ts
@@ -30,6 +30,10 @@ export const isVariable = (term: LambdaTerm): term is Variable => {
   return typeof term === "string";
 };
 
-export const parse = (term: string, library = false) => {
-  return peggyParser.parse(term, { peg$library: library });
+export const parse = (
+  term: string,
+  allowUnderscores = false,
+  library = false
+) => {
+  return peggyParser.parse(term, { peg$library: library, allowUnderscores });
 };