Consistent source code indentation is crucial for code readability, even in free-form languages that are oblivious to whitespace. However, as in a free-form language programmers can format the code as they desire, this may lead to mismatched indentation styles. Automatic code formatters address this problem by rewriting code to a standard layout, however they impose a specific style and only work for syntactically valid code.
We describe an extension of Parsing Expression Grammars (PEGs) that can model indentation information and we show that it can be used to specify different indentation styles. A parser based on such extension can check for indentation inconsistencies and inform the developer about them. To evaluate our approach, we also implemented a full parser for the Lua programming language and used it to parse a well-known Lua codebase. We observed that 98% of source code lines were well-indented according to our specification.