Escape backslashes at the end of a character class option string

Author: hlysine

README.md

@@ -362,7 +362,8 @@ at the beginning and end of each string in the list, so `` charIn`a-` `z` `` mat
 Apart from `-`, `^` and `]` are also escaped in the character class, so you cannot negate a `charIn` via a `^` character
 (you should use `notCharIn`), and you cannot close a character class prematurely.
 
-All other characters are not escaped in `charIn`, so you can use escape sequences such as `\uffff` and `\xff` freely.
+Backslashes `\` are only escaped at the end of a string, so you can use escape sequences such as `\uffff` and `\xff` freely.
+If you want to include `\` in the character class, you should write it at the end of a string or escape with `\\`.
 
 ### Anchors
 

src/expression.ts

@@ -1211,8 +1211,9 @@ export const unicode = r.unicode;
  * Apart from `-`, `^` and `]` are also escaped in the character class, so you cannot negate a `charIn` via a `^`
  * character (you should use `notCharIn`), and you cannot close a character class prematurely.
  *
- * All other characters are not escaped in `charIn`, so you can use escape sequences such as `\uffff` and `\xff`
- * freely.
+ * Backslashes `\` are only escaped at the end of a string, so you can use escape sequences such as `\uffff` and
+ * `\xff` freely. If you want to include `\` in the character class, you should write it at the end of a string or
+ * escape with `\\`.
  *
  * @example
  *

src/modifiers/CharacterClassModifier.ts

@@ -13,7 +13,13 @@ function escapeForCharClass(option: string): string {
     if (charEscaped) {
       charEscaped = false;
     } else if (char === '\\') {
-      charEscaped = true;
+      if (i === option.length - 1) {
+        // escape backslash at the end of the string
+        option = option + '\\';
+        break;
+      } else {
+        charEscaped = true;
+      }
     } else if (char === ']' || char === '^') {
       // escape this character
       option = option.substring(0, i) + '\\' + option.substring(i);

src/types.ts

@@ -666,8 +666,9 @@ export interface RegExpToken {
    * Apart from `-`, `^` and `]` are also escaped in the character class, so you cannot negate a `charIn` via a `^`
    * character (you should use `notCharIn`), and you cannot close a character class prematurely.
    *
-   * All other characters are not escaped in `charIn`, so you can use escape sequences such as `\uffff` and `\xff`
-   * freely.
+   * Backslashes `\` are only escaped at the end of a string, so you can use escape sequences such as `\uffff` and
+   * `\xff` freely. If you want to include `\` in the character class, you should write it at the end of a string or
+   * escape with `\\`.
    *
    * @example
    *

test/functions.test.ts

@@ -930,6 +930,10 @@ describe('charIn', () => {
     expect(charIn`[]`.toString()).toBe('[[\\]]');
     expect(charIn`^a[b]`.toString()).toBe('[\\^a[b\\]]');
     expect(charIn`(])`.toString()).toBe('[(\\])]');
+    expect(charIn`\\`.toString()).toBe('[\\\\]');
+    expect(charIn`\\\\`.toString()).toBe('[\\\\]');
+    expect(charIn`a-z\\``s`.toString()).toBe('[a-z\\\\s]');
+    expect(charIn`a-z``\\s`.toString()).toBe('[a-z\\s]');
   });
   it('is chainable and quantifiable', () => {
     expect(charIn`a``b``c`.char.toString()).toBe('[abc].');