Adds a validator to the search field

src/main/resources/l10n/JabRef_en.properties

@@ -1399,7 +1399,7 @@ should\ contain\ a\ four\ digit\ number=should contain a four digit number
 should\ contain\ a\ valid\ page\ number\ range=should contain a valid page number range
 No\ results\ found.=No results found.
 Found\ %0\ results.=Found %0 results.
-Invalid\ Regex=Invalid Regex
+Invalid\ regular\ expression=Invalid regular expression
 plain\ text=plain text
 This\ search\ contains\ entries\ in\ which\ any\ field\ contains\ the\ regular\ expression\ <b>%0</b>=This search contains entries in which any field contains the regular expression <b>%0</b>
 This\ search\ contains\ entries\ in\ which\ any\ field\ contains\ the\ term\ <b>%0</b>=This search contains entries in which any field contains the term <b>%0</b>