Why do regexes use `$` and `^` as line anchors? (2024) • Hacker News Board

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#11

Post by franky47 » Tue, Jan 21, 2025, 7:15 PM UTC

jcranmer wrote: post #9

franky47 wrote: post #6

My AZERTY keyboard conveniently has them side by side (^ left of $). Could it be a similar reason?

AZERTY is primarily a French keyboard layout, to my knowledge. The standard QWERTY layout for the number key row is `/~, 1/!, 2/@, 3/#, 4/$, 5/%, 6/^, 7/&, 8/*, 9/(, 0/), -/_, =/+. I don't know how far back the mapping of shift keys for the numbers go, but I'd be shocked if there was any around the 1960s or 1970s that put them like your AZERTY keyboard.

Yes (I am French). Those characters are found on the top letter row: AZERTYUIOP^$

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#12

Post by tarkin2 » Tue, Jan 21, 2025, 7:17 PM UTC

Everything starts with someone pointing at something. Everything ends with dollars.

That's how I remembered them anyhow

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#13

Post by red_admiral » Tue, Jan 21, 2025, 7:17 PM UTC

> But also, $ is so important in business that every typewriter has one.

Some of the older Baudot code ones actually had a £ but not a $ symbol. After we'd decided that 5-key chorded keyboards were not the way forwards, and made QWERTY ones, we still had this encoding to deal with: https://hackaday.com/2015/09/27/demonstrating-baudot-code/

The US version then put the $ sign on what we'd today call ^D, which originally was ENQ (a code that still exists in ascii today, and was sort of the pre-TCP version of a SYN).

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#14

Post by milliams » Tue, Jan 21, 2025, 7:19 PM UTC

I remember when learning regexes, struggling to remember which of ^ and $ were the beginning or end of the line. I know that my (and many other) shells used $ as the prompt symbol - i.e. at the beginning of what I was to type in the terminal. It being at the beginning of that line made it easy for me to remember that it also marks the beginning of regexes.

Of course I was 100% incorrect as $ is in fact the end-of-line marker so now I just remember that "it's the opposite"!

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#15

Post by teddyh » Tue, Jan 21, 2025, 7:23 PM UTC

IIRC, the original ASCII standard had ↑ and ← but later changed them to ^ and _, respectively.

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#16

Post by NotAnOtter » Tue, Jan 21, 2025, 7:28 PM UTC

So basically, they used them because they needed to assign them to something ang they knew type writers would have those characters due to their use in business & math.

I mean. Fair enough. I guess.

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#17

Post by acegopher » Tue, Jan 21, 2025, 7:29 PM UTC

At least for ^ I was always told that the ADM3a terminal keyboard had a HOME key with ~ and ^ on it, which is why ~ means "home directory", and ^ means "home of the line".

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#18

Post by magicalhippo » Tue, Jan 21, 2025, 7:29 PM UTC

milliams wrote: post #14

I remember when learning regexes, struggling to remember which of ^ and $ were the beginning or end of the line. I know that my (and many other) shells used $ as the prompt symbol - i.e. at the beginning of what I was to type in the terminal. It being at the beginning of that line made it easy for me to remember that it also marks the beginning of regexes. Of course I was 100% incorrect as $ is in fact the end-of-lin…

> so now I just remember that "it's the opposite"

"No, the other left..."

For me it was easier to remember what ^ was, as it looks like a pointer of sorts, so felt natural it would be the beginning. Like how a string variable points to the first character.

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#19

Post by NotAnOtter » Tue, Jan 21, 2025, 7:30 PM UTC

milliams wrote: post #14

I remember when learning regexes, struggling to remember which of ^ and $ were the beginning or end of the line. I know that my (and many other) shells used $ as the prompt symbol - i.e. at the beginning of what I was to type in the terminal. It being at the beginning of that line made it easy for me to remember that it also marks the beginning of regexes. Of course I was 100% incorrect as $ is in fact the end-of-lin…

Idk why anyone would ever "learn regex's" without the use of a look up table. It's not like timestables where having it memorized will really accelerate other aspects of life. Knowing what ^ does by memory will almost never benefit you.

Re: Why do regexes use `$` and `^` as line anchors? (2024)

#20

Post by aylmao » Tue, Jan 21, 2025, 7:37 PM UTC

milliams wrote: post #14

I remember when learning regexes, struggling to remember which of ^ and $ were the beginning or end of the line. I know that my (and many other) shells used $ as the prompt symbol - i.e. at the beginning of what I was to type in the terminal. It being at the beginning of that line made it easy for me to remember that it also marks the beginning of regexes. Of course I was 100% incorrect as $ is in fact the end-of-lin…

Now that you mention it, I weirdly never struggled with this. I wonder if it's because I first started using the terminal on macOS, which I think by default starts prompts with % instead of $ or >.

Trying to consciously think about the gut-feeling that makes me remember this, all I can come up with is that $ feels "heavier" than ^, which just leads it to naturally feel like the end of a line. Perhaps its verticalness makes it feel like the vertical blinking cursor, and that's what this gut-feeling is really about? I'm not sure, the mind is a mystery sometimes haha.