Regular expressions shenanigans

I’m working on an app for exploring the Dewey Decimal System. It’s based on three-digit numbers in which each number is a different level of specificity. The 700’s are arts, 770’s are photography, 774 is holography. I started with a text file with all the codes and their descriptions and made a flat array of objects like

  { "number": "000", "description": "Generalities " },
  { "number": "001", "description": "Knowledge " },
  { "number": "001.5", "description": "Communications " },

Which worked fine for searching, but we also want to browse. That is, see the 10 top categories, select one, see its subcategories, etc. So what’s required is a tree of nested objects. This page has the codes we need, and if we copy paste and do a little cleanup we get a nicely indented list like so:

000 Generalities
  000 Generalities & computer science
    001 Knowledge
      001.1 Intellectual Life
      001.2 Scholarship and Learning
      001.3 Humanities
      001.4 Research
      001.9 Controversial knowledge
    002 The book
    003 Systems

Which we need to convert to JSON, with every line looking like the code above, except that indented items are an array within the object above them, like so:

  { "number": "000", "description": "Generalities", "subordinates": [
    { "number": "000", "description": "Generalities & computer science", "subordinates": [
      { "number": "001", "description": "Knowledge", "subordinates": [
        { "number": "001.1", "description": "Intellectual Life" },
        { "number": "001.2", "description": "Scholarship and Learning" },
        { "number": "001.3", "description": "Humanities" },
        { "number": "001.4", "description": "Research" },
        { "number": "001.9", "description": "Controversial knowledge" }
      { "number": "002", "description": "The book" },
      { "number": "003", "description": "Systems" },

We’re doing regular expression search and replace in Visual Studio Code. I’m a RegExp amateur, but I did it in three steps like this:

Step 1: replace each line with object notion, preserving indentation:

Search:  ^( *)([0-9]{3}\.*[0-9]*) ?(.*)$
Replace: $1{ "number": "$2", "description": "$3" },

Step 2: Here’s the tricky bit. Starting from the outside and working in, we look for bits indented by 4 or more spaces, replace the object close on the previous line and replace it with a new property and an array opening and insert the array and object close afterwards:

Search:   },\n((   +{ "number": "[0-9.]*", "description": "[A-Za-zæçé 0-9\.&\(\)/\';,\[\]-]*" },\n)+)
Replace: , "subordinates": [\n$1  ]},\n

We’ve got three levels of indents, so we need to repeat this three times, each time increasing the number of spaces (in the search after the (( and in the replace after the $1) by two. (It’s easier than it looks to write these because the editor highlights matches.)

Step 3: Now all we need to do is remove the commas after the final item in ever array, which is invalid JSON:

Search:  ,\n( +)]
Replace: \n$1]

February 05, 2019 RegEx

Written by Alesh Houdek, a web developer in Colorado. This is mostly code snippets and ideas I might want to reuse. HomepageTwitter