The "irregular" form is the positive imperative which uses the stressed pronoun toi after the verb.

Lève-toi !

All other French tenses use the unstressed reflexive pronoun te:

Tu te lèves
Tu ne te lèves pas
Tu t’es levé
Ne te lève pas !
…

It’s a pattern among romance languages imperatives:

Spanish: ¡Levantate! / ¡No te levanta!
Italian: Alzati! / * Non ti alzare! (but also Non alzarti)
Catalan: Aixeca’t! / No t’aixequis!
Romanian: Ridică-te! / Nu te ridica!

In spoken French, ne is often dropped leading to:

Te lève pas !

A more radical evolution builds the negative imperative from the positive :

Lève-toi pas !

This last form is probably not in your French grammar book, though.