Add support for single character variable

[adimiz1]

Brief Summary of Changes

Fix for single character variable (for example: $a) Add tests for single character variable , multiple characters variable and no character variable.

What does this PR address?

• [ ] GitHub issue (Add reference - #XX)
• [ ] Refactoring
• [ ] New feature
• [x] Bug fix
• [x] Adds more tests

Are tests included?

• [x] Yes
• [ ] No

Reviewer, please note:

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I ran the full test suite before pushing the changes and all the tests pass.