How would I match nested "if" statements in regex? (using ruby)

Question

I'm using ruby. I'm looking to match the outer "if" group of this:

{#if x > 10}
    <p>{x} is greater than 10</p>
    {#if 5 > x}
        <p>{x} is less than 5</p>
    {/if}
{/if}

What I have so far is this regex:

\{#if(.*)}(.|\n)*?\{\/if\}

But that just matches the first /if statement instead of the last one (see https://rubular.com/r/4l89GlANV9AVCQ). Any ideas?

(I tried this recursive regex matcher I saw on Stack Overflow: https://rubular.com/r/7a4PYTtxFQftob, but I'm struggling to make it work for the above)

blhsing · Accepted Answer · 2021-09-06 03:08:35Z

2

It's basically the same as the regex for balanced parentheses that you linked to, but since the markers here are of multiple characters rather than single characters, you would need to use a negative lookahead pattern instead of simply a negated character class to ensure a matching character in the middle isn't part of the markers for the balanced construct:

{#if.*?}(?>(?!{(?:#if.*?|\/if)}).|\g<0>)*{\/if}

Demo: https://rubular.com/r/7BU2FStPEP34W4

Note that you should enable MULTILINE mode with the m option in order for . to match a newline character.

edited Sep 6, 2021 at 3:08

answered Sep 6, 2021 at 2:43

blhsing

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1 Comment

sjsc Over a year ago

This is perfect!! I'm studying more into negative lookaheads now. I was racking my head for hours on this and your solution solved it. Thank you so much bhlsing! :) :)

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