Formik, Yup Password Strength Validation with React

You need to pass an actual RegExp object to matches, not a string. Just replace the double quotes with forward slashes in your password schema:

EDIT: Updated to use regex from @Bren

password: yup
    .string()
    .required('Please Enter your password')
    .matches(
      /^(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9])(?=.*[!@#\$%\^&\*])(?=.{8,})/,
      "Must Contain 8 Characters, One Uppercase, One Lowercase, One Number and One Special Case Character"
    ),