The ABNF rule for searchWord is overly restrictive, allowing only Unicode letters, numbers, and some punctuation characters. Character sequences with other characters need to be enclosed in double-quotes to pass the ABNF, i.e. be formulated as a searchPhrase.

However double-quoted "phrases" typically have special meaning for search engines, e.g. use exact, non-fuzzy match, meaning that $search=word and $search="word" have different results.

Also when used for type-ahead the search expression will typically be incomplete while the user is typing, and the user's expectation is to already see matching proposals while typing. All these search requests should give a search result and none of them should lead to an error:

$search="
$search="b
$search="bl
$search="blu
$search="blue
$search="blue%20
$search="blue%20p
$search="blue%20pi
$search="blue%20pil
$search="blue%20pill
$search="blue%20pill"

 Same for unbalanced parentheses.