Is the word ex the same in Brazilian and European Portuguese?

Yes

The word "ex" is identical in meaning, spelling, and grammatical function in both Brazilian and Continental Portuguese. It serves as both a prefix (e.g., ex-namorado, ex-presidente) and as a standalone noun to refer to a former partner (e.g., o meu ex). The only difference lies in pronunciation: in some Brazilian regions, the "x" may be articulated with a slightly different sibilance or vowel emphasis, whereas in Portugal, the pronunciation is typically more standardized as [ɛks].

Brazilian Portuguese Examples:

1. Eu vi meu ex no shopping ontem. (I saw my ex at the mall yesterday.)
2. O meu ex é muito gente boa. (My ex is a very nice person.)
3. Ela terminou com o ex dela. (She broke up with her ex.)
4. Não quero falar do meu ex agora. (I don't want to talk about my ex right now.)
5. Meu ex me ligou tarde da noite. (My ex called me late at night.)

Continental Portuguese Examples:

1. Eu vi o meu ex no centro comercial ontem. (I saw my ex at the shopping mall yesterday.)
2. O meu ex é uma pessoa muito porreira. (My ex is a very cool person.)
3. Ela terminou com o ex dela. (She broke up with her ex.)
4. Não quero falar do meu ex agora. (I don't want to talk about my ex right now.)
5. O meu ex ligou-me tarde da noite. (My ex called me late at night.)