Mehul Kar

19 May, 2017

Regex: The Greedy Dot Star

programming

I was recently extending redcarpet (a Markdown rendering library) to render a CHANGELOG.md in a nicer way. The idea is that markdown like this:

- [fixed] that one bug
- [added] that one feature
- [removed] that other thing
- this one doesn't have a tag

turns into this HTML:

<li><span class='fixed tag'>fixed</span> that one bug</li>
<li><span class='added tag'>added</span> that one feature</li>
<li><span class='removed tag'>removed</span> that other thing</li>
<li>this one doesn't have a tag</li>

Notice that the main transformation is the “tag” inside square brackets is turned into a span element with a class of the same name.

Extending the rendering of list elements in redcarpet specifically involves overwriting a method that looks like this1:

def list_item(text)
end

The text argument in the method signature is a single list item, stripped of the identifier (in this case “-”). So, inside this method, I have to parse [fixed] that one bug and turn it into my desired HTML.

First Regex Approach

My first regex approach was to capture everything inside [], and use the captured group to create the span tag. This seemed straightforward:

def list_item(text)
  matched = text.match(/^\\[(.*)\\] (.*)$/)
  tag_type = matched[1]
  note = matched[2]

  "<li><span class=#{tag_type} tag>#{tag_type}</span>#{note}</li>"
end

The important part of this regex is \\[(.*)\\]2.

  • The double backslashed [ and ] characters match literal square brackets in my string
  • The (.*) between the square bracket literals match ALL characters inside the square brackets

The second part of the regex (.*) simply captures the rest of the string in another capture group that we can assign to the note variable.

(I’ve omitted the handling of list items that don’t contain square brackets for the sake of simplicity.)

The Edge Case

This solution worked pretty well, but I ran into an issue. When I attempted to render a list item that contained a second pair of [] literals, the HTML output was all messed up.

For example, this markdown:

- [fixed] [something] happened here

SHOULD be rendered as:

<span class='fixed tag'>[something] happened here</span>

But instead, I was getting:

<li><em class='fixed] [something tag'>fixed] [something</em> <span>happened here</span></li>

At first glance, this looked like garbage text to me, but then I realized that the first capture group of my regex, between the [] literals, was capturing from the first [ to the last ] in the input string.

So, my first capture group was capturing: fixed] [something instead of just fixed.

This was strange to me, because I assumed that the .* in \\[(.*)\\] would capture UNTIL it reached the first ].

But it turns out that the .* is “greedy”. That means that that it will match all characters to the end of the entire string, and then work backwards to find the next match for the remaining regex.

To break it down.. here’s what happens:

  1. \\[ finds the first literal open square bracket
  2. (.*) captures the rest of the string
  3. \\] starts at the end of the string and goes backwards one character at a time until it finds a literal close square bracket.
  4. (.*) takes the remaining string after that closing square bracket and captures that in a group.

So in step 3, the wrong square bracket is found :(.

Solution

There are two ways to fix this:

Make .* NOT greedy

The first way is to change the behavior of .* so that it doesn’t gobble up the entire string, like the greedy monster it is. So I can change the regex matcher to:

\\[(.*?)\\]

Notice the extra ?.

Although this works, the way this Regex has to work now is instead of capturing a bunch of the string, it has go character by character until it finds the next closing ] literal. This makes the regex a lot slower. Since I was rendering user submitted CHANGELOG.md files, I didn’t want to rely on the size of user text being small.

Exclude [] literals from the first .*

Another solution is to exclude [] literal square brackets from the first (.*) capture group:

\\[([^\\[\\]]*)\\]

This looks confusing to read, but if you break it down, it makes sense:

  1. \\[ marks the opening literal square bracket
  2. ( marks the beginning of the capture group
  3. [ marks the beginning of a list
  4. ^ means NOT
  5. \\[\\] are again the literal square bracket characters
  6. ] is the end of the list
  7. * means any number of characters
  8. ) marks the end of the capture group
  9. \\] marks the ending literal square bracket

Numbers 2-8 in this list are the replacements for a (.*) and mean “capture 0 or more characters that are not literal square brackets in a group”.

So now, my regex will match my “tag” after the first open square bracket until the first closing square bracket.

The final regex is:

^\\[([^\\[\\]]*)\\] (.*)$

Note that this will still break in some (many) cases, but I’m not addressing those here.

Links

Here’s some links with the regex explained here:

Footnotes

  1. I’ve omitted the second argument that allows handling of ordered and unordered lists for simplification.
  2. I’m not entirely clear in regex land when double backslashes are needed for escaping and when a single backslash will do. For example, the regex101 links at the bottom of this article have regexes with only single backslashes for escaping.

Want to talk about this blag? Email me or send me a toot @mehulkar!