Instead of saying, "they are the same in syntax", how to say it the other way, e.g., "they are *syntaxly* the same"?
http://www.tfd.com/syntax has no entry of its adverb form either. Thanks.
according to the rules of syntax
Remember that the adverb-forming suffix -ly is usually attached to adjectives rather than nouns. The adjectives corresponding to syntax are syntactic and syntactical.
Whenever there's a pair of collateral adjectives ending in -ic and -ical, the corresponding adverb ending in -ically is preferred over the one with -icly even if the adjective ending in -ic is more common.