regularo

Decode it and you are done. Regulo is a rule, at rank 759. -Ar- is the suffix for a collection considered as one thing. A regularo is therefore the whole body of rules, taken together: a rulebook, a set of regulations, a constitution's operating half. You do not translate the pieces separately in your head after the first few times, but you can, and that is the point.

The -ar- pattern, once, so it sticks. The suffix turns a countable unit into the mass or set it belongs to, and the results are usually the word English needs a different noun for:

• arbo a tree, arbaro a forest
• vorto a word, vortaro a dictionary
• homo a person, homaro humanity
• ŝtupo a step, ŝtuparo a staircase
• regulo a rule, regularo the regulations

Note how uneven the English column is: forest, dictionary, humanity, staircase, regulations. Five unrelated words for one Esperanto operation. This is the strongest single argument the affix system has, and -ar- is where a learner usually first believes it.

The Fundamento and its regularo, because you will meet this. Esperanto has a constitutional document, the Fundamento of 1905, which fixed the grammar and a core vocabulary and declared them untouchable. Its sixteen rules are la Fundamenta gramatiko; the standing rules under which the Akademio and the associations actually operate are their regularoj, which can be amended and regularly are. The distinction is worth holding on to: the fundamento is the thing that does not change, the regularo is the thing that governs a body's day-to-day work. Every local club you join will have one, and reading it is a surprisingly good way to meet formal Esperanto.

Neighbours. Reguligi is to regulate, to bring something under rules; regula is regular in the sense of conforming to rule, though regula and regulara both circulate and laŭregula is clearer than either. Keep regi (to rule, to govern) in a separate box: it is the same Latin family but a different Esperanto root, and regularo has nothing to do with governing a country.