Thanks again for this answer. I modified your regex a bit in order to work correctly with the libreoffice regex function. I added .* twice, at the beginning and at the end of the regex.

With blah blahTESTblab la14blah-15S reblain cell A1 :

• =REGEX(A1;"(?i)(TEST)(?:.*(\d{1,2}S)|)";"★$1$2") gives blah blah★TEST15S rebla
• =REGEX(A1;"(?i).*(TEST)(?:.*(\d{1,2}S)|).*";"★$1$2") gives ★TEST15S