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.

In reply to Re^3: Help with multiple line regex subsitution by Porculus
in thread Help with multiple line regex subsitution by gator456

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