嵌套相同条件时正则表达式不起作用

Question

Background: I'm just fiddling around with an idea for simple templating which only provides if/for/render, to see how feasible it is and if it makes sense to use in my personal project. As opposed to using NVelocity or Razor, or anything else.

I've written a regular expression:

(?:(?:(?<open>\[if (?<if>[a-zA-Z0-9\.]+)\])(?<content>[^\[]*))+(?:[^\[]*(?<close-open>\[end if\]))+)+(?(open)(?!))

And when used with the sample text:

<div>
[if variable3]{{variable3}}[end if]
</div>

<div>
[if variable1]

    {{variable1}}

    [if variable2]
        <br>
        {{variable2}}
    [end if]

[end if]
</div>

It's working as expected. I get 2 matches, and if the 2nd match is valid I can parse the inner capture.

Problem is if i have multiple nested matches. So given:

<div>
[if variable3]{{variable3}}[end if]
</div>

<div>
[if variable1]

    {{variable1}}

    [if variable2]
        <br>
        {{variable2}}
    [end if]

    [if variable4]
        <br>
        {{variable4}}
    [end if]

    [if variable5]
        <br>
        {{variable5}}
    [end if]

[end if]
</div>

What I end up with is the first capture being correct, and then all 3 individual captures and not the outer one for the 2nd match.

If I expand the capture to ignore \\[ for the inner content, it causes the first and second match to combine into a single match. :(

Does anyone know how to fix this? (and if you have a better idea of how to do this templating would be keen to know in the comments)

Answer 1

You may use

@"(?s)\[if\s+(?<if>[^][]+)](?<fullBody>(?>(?:(?!\[if\s|\[end\ if]).)+|(?<-open>)\[end\ if]|(?<open>)\[if\s+(?<if>[^][]+)])*(?(open)(?!)))\[end\ if]"

See the regex demo .

Details (note that you may use it inside C# code due to the x modifier):

@"(?sx)               # Singleline and IgnorePatternWhitespace flags on
  \[if\s+             # "[if" and then 1+ whitespaces
    (?<if>[^][]+)     # "If" group: one or more chars other than "]"
  ]                   # a "]" char
   (?<fullBody>       # Group "fullBody" containing all nested if blocks
     (?>              # Start of an atomic group
       (?:(?!\[if\s|\[end\ if]).)+|    # any char, 1+ occurrences, that does not start the "[if " or "[end if]" substring, or...
       (?<-open>)\[end\ if]|           # "[end if]" substring and an item is popped from Group "open", or
       (?<open>)\[if\s+(?<if>[^][]+)]  # Group "open": "[if", 1+ whitespaces, Group "if": 1+ chars other than "[" and "]", and then a "]" char
     )*               # repeat atomic group patterns 0 or more times
     (?(open)(?!))    # A conditional: if Group "open" has any items on its stack, fail and backtrack
   )                  # End of fullBody group
  \[end\ if]"         # "[end if]" substring

If you do not care if an if block is nested in which block, you may plainly get a full list of if blocks using a variation of this regex:

var pattern = @"(?s)(?=(?<ifBlock>\[if\s+(?<if>[^][]+)](?<fullBody>(?>(?:(?!\[if\s|\[end\ if]).)+|(?<-open>)\[end\ if]|(?<open>)\[if\s+(?<if>[^][]+)])*(?(open)(?!)))\[end\ if]))";

The pattern above is just wrapped with another named capturing group and is placed inside a positive lookahead. While the match value will always be empty, groups will hold all the values you may need.