You can find basic information about How Regular expressions can be used in Siebel in below links,
http://siebeldev.blogspot.com/2009/04/regular-expression-validation.html
http://download.oracle.com/docs/cd/B40099_02/books/eScript/eScript_JSReference252.html
For the Non English character set support, RexExp pattern should be writed with Unicode characters.
You can get information about Unicode below link,
You can find a simple pattern about Turkish First Name validation below,
“^[\u0041-\u005A]{1}\u002E?[\u0041-\u005A|\u0061-\u007A|\u0130|\u0131|\u00D6|\u00F6|\u00DC|\u00FC|\u00C7|\u00E7|\u011E|\u011F|\u015E|\u015F]*$”
Pattern explanation:
First character should be Upper Case(\u0041-\u005A]{1})
Second character could be “.” (\u002E?)
Other characters can be anything in Turkish Alphabet([\u0041-\u005A|\u0061-\u007A|\u0130|\u0131|\u00D6|\u00F6|\u00DC|\u00FC|\u00C7|\u00E7|\u011E|\u011F|\u015E|\u015F])