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https://buttondown.email/hillelwayne/archive/why-do-regexes-use-and-as-line-anchors/
Last week I fell into a bit of a rabbit hole: why do regular expressions use $ and ^ as line anchors?1
This talk brings up that they first appeared in Ken Thompson's port of the QED text editor. In his manual he writes: b) "^" is a regular expression which matches character at the beginning of a line.
c) "$" is a regular expression which matches character before the character (usually at the end of a line)
QED was the precursor to ed, which was instrumental in popularizing regexes, so a lot of its design choices stuck.
Okay, but then why did Ken Thompson choose those characters?
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Why Do Regexes Use `$` and `^` as Line Anchors?
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Q: How many IBM CPU's does it take to do a logical right shift?
A: 33. 1 to hold the bits and 32 to push the register.
(Score: 2) by VLM on Thursday March 28 2024, @04:32PM
At some point it turns into "lets scrap it and replace with a simple parser"
It would be interesting to see a compiler that uses regex instead of a parser design.
You can regex a simple machine code assembler, but much more than the simplest and forget regex design it's parser time. Its pretty easy to make an assembler that uses regex to assemble a "nop" but "movb r0 (r1)+" (Its macro-11) would take a thundering lot of regex. That assembly language would, when thought about like C, be like write the contents of the first byte of variable/register r0 to memory using r1 as the pointer then increment the pointer for later use, essentially strcpy a single type char variable into a char array and get ready to copy the next char. C of course is just slightly fancied up PDP11 assembly, it's only on inferior processors that C looks like a separate language.