Use the language parser/grammar (e.g., Langium/Jison) for diagram syntax instead of hand-rolled regex parsing. Regex is allowed only for small, well-bounded token-level helpers when (a) it precisely matches the intended grammar slice (e.g., remove only *outer* quotes), (b) it doesn’t destructively change user content or semantics, and (c) it preserves source positions/ranges by avoiding pre-processing that removes/reflows lines before parsing. Apply these rules: - **No “core syntax” regex parsing**: if parsing needs to understand quoting, delimiters, comments/directives, or multi-token constructs, implement it in the `.langium` / grammar layer. - **No destructive transforms**: transformations like stripping wrappers must not remove inner quotes/apostrophes. - **Parser-owned comment/directive handling**: handle `%% ...` comments and directives in the parser so AST line/range data stays accurate. - **Delimiter ownership is explicit**: commas/other delimiters should be represented in grammar structure; avoid regexes that “capture beyond” node/value boundaries. Example: wrapper-quote removal must be outer-only (not global): ```ts function stripOuterQuotes(s: string): string { const t = s.trim(); if ((t.startsWith('"') && t.endsWith('"')) || (t.startsWith("'") && t.endsWith("'"))) { // remove only the outer quotes return t.slice(1, -1); } return t; } ``` When you must match with regex (e.g., token-level extraction), keep it narrowly scoped and back it with tests for edge cases (escaped quotes, whitespace variations, delimiter presence, comments).