Can you give an example of a loop in an algebraic syntax please? (All I can think of is CFG style, like "A -> (aA)?" oh, of course, there's also "a*" - still,

Can you give an example of a loop in an algebraic syntax please? (All I can think of is CFG style, like "A -> (aA)?" oh, of course, there's also "a*" - still, it would be interesting to see an eg.)