Programming - modifications to text file, looking for keywords and then making numerical alterations

I need a program to act as a post processor for some CNC work. The actual function is not complicated, but might take a little explaining. The background is that I am doing rotary carving on a CNC (computer controlled) woodworking machine. The workpiece slowly and continuously rotates, and the cutting tool very slowly progresses along the model, moving up and down to create the required shape. Effectively it spirals its way up. The issue I have is that the carving is broken into small areas, so that the tool will do a section, and then go back to repeat it once or twice, and then move on again. There are then noticeable artefacts where each section starts/stops. The machine itself is reading a g-code file, which is straight text. I need a program that can read this text file, and modify it to create smooth lead ins and lead out between sections. So, in the g-code each line is a command. The machine has co-ordinates in X, Y, Z and A. Z is the up and down, which will need to be modified to create the lead in/out. Progression is slowly down the Y axis (Y will always be decreasing in value), this will need to be looked at to calculate the modification to Z. A is rotary axis, X side to side - neither will need to be changed for this. The lead in / out will be a simple linear progression. I’ll attach a picture of a very simple model, showing the toolpath for two operations. The step-over, or distance between lines of the spiral, is different between operations, so you can better see which is which, and they also overlap in the middle. This is how I will do it, creating toolpaths that overlap, and then using this program so that they lead in and lead out from the same point on Y. To let the program know what needs changing, I'll add text into the g-code. This will look like (LEADIN Y3 Z2) - so here the program will need to hunt forward, apply +2 at first to Z, continuing to add to Z an amount which changes linearly down to +0 over a distance of Y3. Similarly (LEADOUT Y2 Z1) - will need to hunt backwards, applying +1 to Z and then moving up the file, changing Z values for a Y distance of 2. Because freelancer has a character limit, I cannot include a worked example here, else this description would be too long, so I have attached it as a word file instead. In terms of file size, they typically get up to 20mb. So speed of working through and making modifications, is a factor! I’ll attach the complete g-code file for this simple example. Requirements for the programming language, form etc: - Raw code in whatever language you want to work with is fine for me, so long as I can easily run it myself. I am told python is a good option, so have included it in the skills list, but I dont mind. Alternatively to me running the code, would be a compiled program with a dialog box to load gcode file, but I still need the raw code and ability to compile myself in that case. - This is because I need to be able to modify the function added. At present, description above is purely linear for Z added in terms of Y. I very likely will want to modify this, for instance linear for the first 75% of Y distance, and then exponential decrease for the last 25%. I can code in C++ etc but am not familiar with programs to read and modify text. To aid me in modifying the function, please make a note in the code where this is, or have it as a subroutine I can easily find, or something. Alternatively, if a compiled program then a way for me to enter a function into a dialogue box would be nice - but not necessary, probably total overkill. - I understand the easiest way to modify the code, is to load it all into memory and output a new file, with a slightly modified file name. That works fine for me. Any questions, please let me know!