Invited Talks -- Morphic sequences versus automatic sequences -- Parsimonious Computational Completeness Pointlike sets and separation: a personal perspective -- Regular Papers -- A strong non-overlapping Dyck code -- Active Learning of Sequential Transducers with Side Information about the Domain -- Compositions of Constant Weighted Extended Tree Transducers -- Extremal Binary PFAs in a Cerny Family -- Variations on the Post Correspondence Problem for Free Groups -- Reducing local alphabet size in recognizable picture languages -- Properties of Graphs Specified by a Regular Language -- Balanced-by-construction regular and omega-regular languages -- Weighted Prefix Normal Words: Mind the Gap -- Two-Way Non-Uniform Finite Automata -- Integer Weighted Automata on Infinite Words -- Deciding FO2 Alternation for Automata over Finite and Infinite Words -- State Complexity of Projection on Languages Recognized by Permutation Automata and Commuting Letters -- Constrained Synchronization and Subset Synchronization Problems for Weakly Acyclic Automata -- Lyndon words formalized in Isabelle/HOL -- The Range of State Complexities of Languages Resulting from the Cascade Product-The General Case (Extended Abstract) -- Second-order finite automata: expressive power and simple proofs using automatic structures -- Reversible Top-Down Syntax Analysis -- Symmetry groups of infinite words -- Bounded Languages Described by GF(2)-grammas -- Definability Results for Top-Down Tree Transducers -- The hardest LL(k) language -- Upper Bounds on Distinct Maximal (Sub-)Repetitions in Compressed Strings -- Branching Frequency and Markov Entropy of Repetition-Free Languages -- A Linear-time Simulation of Deterministic d-Limited Automata -- Caratheodory Extensions of Subclasses of Regular Languages -- Parikh Word Representable Graphs and Morphisms.

This book constitutes the proceedings of the 25th International Conference on Developments in Language Theory, DLT 2021, which was held in Porto, Portugal, during August 16-20, 2021. The conference took place in an hybrid format with both in-person and online participation. The 27 full papers included in these proceedings were carefully reviewed and selected from 48 submissions. The DLT conference series provides a forum for presenting current developments in formal languages and automata. Its scope is very general and includes, among others, the following topics and areas: grammars, acceptors and transducers for words, trees and graphs; algebraic theories of automata; algorithmic, combinatorial, and algebraic properties of words and languages; variable length codes; symbolic dynamics; cellular automata; polyominoes and multidimensional patterns; decidability questions; image manipulation and compression; efficient text algorithms; relationships to cryptography, concurrency, complexity theory, and logic; bio-inspired computing; quantum computing. The book also includes 3 invited talks in full paper length. .