expr: '^' should act as a special character at the start of a subexpression

> ^ acts as a special character only at the beginning of the regular expression or subexpression (that is, after \( or \|).

Reference: https://www.gnu.org/software/sed/manual/html_node/BRE-syntax.html

### Examples

- `expr b : 'a\|^b'`
  - Expected: "1"
  - Got: "0"
- `expr ab : '\(^a\)b'`
  - Expected: "a"
  - Got: ""