strange behavior with RLE compression of flash new code

The compression happens after save() and the code should be restored exactly as it was before - there should be no difference between LZ4 and regular RLE.

Maybe you could compare with a recent build that doesn't have RLE compression like this one

Chances are it will be exactly the same as the LZ4 version.

My guess is that the increased memory usage is because the new Espruino builds actually store line numbers for functions (which takes one extra variable per function) in order to give you proper debugging. If you use the latest Web IDE from GitHub I just added an option so you can turn that off if you want to, which should save you some memory.

in the future I'd like to build a version with some functionality to manage regular expression

That'd be great - and if you are using Espruino in your company's products, anything you can contribute back to help Espruino is appreciated.