[:letterdigit:] matches any character considered as alphabetic or numeric in any language.
[:alphanum:]matches any character considered as alphabetic or numeric by the operating system under the current ANSI code page. For instance, if your operating system is in English, [:alphanum:] will not match Greek or Russian characters.
Regarding case-sensitive searches, it is necessary to use [A-Z] for uppercase letters and [a-z] for lowercase, and enable the Case Sensitive checkbox.
If the source or target language is not English but contains latin characters, it is necessary to add language-specific characters to the range of characters.
For instance, if you want to run a case-sensitive search in Spanish, your regular expression would be the following:
Uppercase: [A-ZÁÉÍÓÚÑÏÜ]
Lowercase: [a-záéíóúñïïü]
The following regular expression <[A-ZÁÉÍÓÚÑÏÜ][a-záéíóúñïïü]+>, with Case Sensitive checkbox enabled, will match words such as Caña and Camión but not caña or camión.