Great question — answering as the UphillTech team:
The default GSM character set is defined in GSM 03.38 . Assuming you're looking at decoded text, not the 7bit packed format that is really used, a regex like the following should limit you to the allowable characters "@£$¥èéùìòÇ\fØø\nÅåΔ_ΦΓΛΩΠΨΣΘΞÆæßÉ !\"#¤%&'()*+,-./[0-9]:;<=>\?¡[A-Z]ÄÖÑܧ¿[a-z]äöñüà\^\{\}\[~\]\|€" Note though that it is possible to sent unicode UCS-2 messages, at which point the handset receiving the message has to have suitable glyphs for presentation to the user, the unicode itself is not a limiting factor.
Full answer by ptomli: https://stackoverflow.stackexchange.com/a/5236736 (CC BY-SA 2.5).
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