Question

I’m doing some regular expression gymnastics. I set myself the task of trying to search for C# code where there is a usage of the as-operator not followed by a null-check within a reasonable amount of space. Now I don’t want to parse the C# code. E.g. I want to capture code snippets such as

    var x1 = x as SimpleRes;
    var y1 = y as SimpleRes;
    if(x1.a == y1.a)

however, not capture

    var x1 = x as SimpleRes;
    var y1 = y as SimpleRes;
    if(x1 == null)

nor for that matter

    var x1 = x as SimpleRes;
    var y1 = y as SimpleRes;
    if(somethingunrelated == null) {...}
    if(x1.a == y1.a)

Thus any random null-check will count as a “good check” and hence not found.

The question is: How do I match something while ensuring something else is not found in its sourroundings.

I’ve tried the naive approach, looking for ‘as’ then doing a negative lookahead within a 150 characters.

\bas\b.{1,150}(?!\b==\s*null\b)

The above regular expression matches all of the above examples infortunately. My gut tells me, the problem is that the looking ahead and then doing negative lookahead can find many situations where the lookahead does not find the ‘== null’.

If I try negating the whole expression, then that doesn’t help either, at that would match most C# code around.

Answer 1

I love regex gymnastics! Here is a commented PHP regex:

$re = '/# Find all AS, (but not preceding a XX == null).
    \bas\b               # Match "as"
    (?=                  # But only if...
      (?:                # there exist from 1-150
        [\S\s]           # chars, each of which
        (?!==\s*null)    # are NOT preceding "=NULL"
      ){1,150}?          # (and do this lazily)
      (?:                # We are done when either
        (?=              # we have reached
          ==\s*(?!null)  # a non NULL conditional
        )                #
      | $                # or the end of string.
      )
    )/ix'

And here it is in Javascript style:

re = /\bas\b(?=(?:[\S\s](?!==\s*null)){1,150}?(?:(?===\s*(?!null))|$))/ig;

This one did make my head hurt a little…

Here is the test data I am using:

text = r"""    var x1 = x as SimpleRes;
    var y1 = y as SimpleRes;
    if(x1.a == y1.a)

however, not capture
    var x1 = x as SimpleRes;
    var y1 = y as SimpleRes;
    if(x1 == null)

nor for that matter
    var x1 = x as SimpleRes;
    var y1 = y as SimpleRes;
    if(somethingunrelated == null) {...}
    if(x1.a == y1.a)"""