Grammar-obeying program synthesis is a subfield of program syn- thesis (i.e., the task of automatically discovering an executable piece of code) that requires generated code to follow a Backus-Naur Form (BNF) grammar. Having code that obeys a grammar limits the structure of the code and the set of callable functions/method- s/libraries, thus having an impact on (i) security: e.g., using ma- licious functionalities, blacklisted libraries, and questionable con- structs, (ii) the computing environment: e.g., if the hardware is not able to run some functions or if some functions are too costly in terms of memory or energy, and (iii) code quality: improving code style and readability, reducing code smells, and enhancing consistency. Grammar-Guided Genetic Programming (G3P) is recognised as one of the most successful approaches for grammar-obeying pro- gram synthesis that evolves programs in arbitrary languages to solve program synthesis problems based on a set of input-output examples and predefined BNF grammars. Despite its success, the restriction on the evolutionary system to only leverage input-output error rate during its assessment of the programs it derives lim- its its scalability to larger and more complex program synthesis problems. Alternatively, while large language models (LLMs) are showing increasing success in program synthesis from a task de- scription, they often struggle to generate accurate code due to am- biguity in task specifications, complex programming syntax, and lack of reliability in the generated code. Furthermore, their gener- ative nature limits their ability to fix erroneous code with iterative LLM prompting. In this thesis, I focus on enhancing the performance of G3P for grammar-obeying program synthesis tasks. I begin by evaluating the state-of-the-art systems, including G3P and LLMs, in grammar- obeying program synthesis. Next, I explore how to map LLM- generated code into grammar-obeying forms, followed by seeding the grammar-mapped LLM-generated code into G3P’s initial pop- ulation to improve the evolutionary process. Building on these foundations, I investigate the potential of using code similarity measures to guide the evolution of programs, leading to the devel- opment of Bi-Objective Grammar-Guided Genetic Programming (BOG3P) and Multi-Objective Grammar-Guided Genetic Program- ming (MOG3P), which combine input-output error rate with mul- tiple similarity measures as objective functions. Finally, I integrate LLM-generated code into the MOG3P framework in two ways: (i) using LLM-generated code as a seed for the evolutionary pro- cess through a grammar-mapping phase, and (ii) leveraging multi- ple similarity measures towards LLM-generated code to guide the search process throughout the evolution. Additionally, I further extend the MOG3P framework by incorporating code from multi- ple LLMs to enhance the diversity and effectiveness of the search process.