Parser generation by example for legacy pattern languages

Most modern software languages enjoy relatively free and relaxed concrete syntax, with significant flexibility of formatting of the program/model/sheet text. Yet, in the dark legacy corners of software engineering there are still languages with a strict fixed column-based structure — the compromises of times long gone, attempting to combine some human readability with some ease of machine processing. In this paper, we consider an industrial case study for retirement of a legacy domain-specific language, completed under extreme circumstances: absolute lack of documentation, varying line structure, hierarchical blocks within one file, scalability demands for millions of lines of code, performance demands for manipulating tens of thousands multi-megabyte files, etc. However, the regularity of the language allowed to infer its structure from the available examples, automatically, and produce highly efficient parsers for it.