A common question about HREFLang Elements and page and in the XML site map is are they case sensitive? For example,

 hreflang=”en-gb” vs hreflang=”en-GB or hreflang=”EN-GB”

No they are not case sensitive.

Search engines will accept both. However, the standard syntax does typically show the country/region in upper case for example for the UK it can be hreflang=”en-GB” where the language is in lower case and the country/region in upper case both both versions are accepted.