I learned that most infinitives end in -en, with the exception of irregulars like sein. Is the verb erinnern (to remind) also irregular? Are there any other verbs whose infinitive ends in -ern? If so, what are they — regular or irregular?
The three common infinitive endings of German verbs are -en, -eln, and -ern. As you already wrote, verbs ending in -en are in the far majority. But the other two are also common:
-eln: e.g. handeln, hebeln, grübeln, gründeln, makeln, lächeln, wechseln … "googeln"
-ern: e.g. ackern, ändern, ärgern, blättern, dauern, eiern, feiern, flimmern, rudern, stochern
The complete list would be pages long. To help you making a guess what the infinitive is, these verbs often have a related noun ending in -el or -er.