Re^3: Help with multiple line regex subsitution

Caution: this approach does not scale well to files of more than several megabytes (depending on your system's memory).

In the unlikely case that this is a problem, it's fairly straightforward to switch to reading blocks rather than slurping the entire file in one go.

You just have to remember to handle the case where a sequence you want to match is split across a block boundary, and that's just a case of checking to see if the last part of the block could possibly be a prefix of a matching sequence, and if it could, then prepending it to the next block you read. (Implementing this is left as an exercise for someone in a time zone where it's earlier in the day.)

But it's really very unlikely that this will be a problem. A file with a format like that is unlikely to reach even one megabyte -- if it does, then either it should have been split up into multiple files long ago (if it's intended for human reading and writing); or it should be being stored in a standard format like XML or JSON, and transformed with a standard library, not regular expressions.

Comment on Re^3: Help with multiple line regex subsitution

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Re^4: Help with multiple line regex subsitution by AnomalousMonk (Archbishop) on Apr 15, 2009 at 16:43 UTC
In the unlikely case that this is a problem, it's fairly straightforward to switch ... I think we are not so far apart on this. Switching involves abandoning one approach to solving the problem (and I might more clearly have expressed the idea that 'this approach' meant 'read the entire file to a string, operate on the string, and write the string back to a file') for a significantly different approach, perhaps one of those outlined in the balance of your reply. This is what I mean when I write that the approach does not scale well. I agree that, given what one can divine about the OPer's data from the OP, the problem is not likely to arise. In fact, I would go further and say that, in my experience, problems like this never happen – until they happen, and then you have to go back and change a bunch of stuff and re-test everything; which, while straightforward, still costs time, effort and risk; hence, the caution about lack of scalability.	[reply]

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Re^4: Help with multiple line regex subsitution
by AnomalousMonk (Archbishop) on Apr 15, 2009 at 16:43 UTC

In the unlikely case that this is a problem, it's fairly straightforward to switch ...

Switching involves abandoning one approach to solving the problem (and I might more clearly have expressed the idea that 'this approach' meant 'read the entire file to a string, operate on the string, and write the string back to a file') for a significantly different approach, perhaps one of those outlined in the balance of your reply. This is what I mean when I write that the approach does not scale well.

I agree that, given what one can divine about the OPer's data from the OP, the problem is not likely to arise. In fact, I would go further and say that, in my experience, problems like this never happen – until they happen, and then you have to go back and change a bunch of stuff and re-test everything; which, while straightforward, still costs time, effort and risk; hence, the caution about lack of scalability.

[reply]