Sharp Two-Round Adaptivity and Round Hierarchies for Semantic Regular Expressions
Semantic regular expressions (SemREs) attach external Boolean predicates to matched spans, making both the number and the sequentiality of oracle calls central resources. For a fixed expression and word, we represent membership by a polynomial-size monotone span circuit and identify optimal semantic evaluation with Boolean decision-tree evaluation. We determine the extremal power of adaptivity asymptotically sharply. For every $E\ge2$, there is a unary, star-free, semantic-depth-one instance of syntax size $\Theta(E)$ with $E$ essential oracle keys and only unit-length semantic spans whose one-round cost is $E$, whereas its exact two-round and unrestricted deterministic costs are \[ \log_2 E+\tfrac12\log_2\log_2 E+O(1). \] Consequently, the largest nonadaptive-to-adaptive ratio is $(1+o(1))E/\log_2E$, including the optimal leading constant. A second restricted family exhibits a complete round hierarchy: its optimal $R$-round cost is $\Theta(R E^{1/R})$. Thus the maximal gap already appears in two rounds, while other instances interpolate smoothly across all round budgets. Both constructions admit one-predicate realizations over the fixed alphabet $\{0,1,\#\}$ with logarithmic-length semantic spans and $O(E\log^2 E)$ total representation size. Under pointwise error $\delta<1/2$ and worst-case expected cost, randomized nonadaptive complexity is exactly $(1-2\delta)E$ for every instance with $E$ essential keys. Finally, for a fixed word $w$ and $h$ predicate names, the exact randomized minimax value is $(1-2\delta)h sd(w)$, where $sd(w)$ counts distinct substring values; a span bound $s$ replaces $sd(w)$ by $sd_s(w)$. These results separate semantic information acquisition, parallel latency, and local symbolic matching cost.