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Why Do Regexes Use `$` and `^` as Line Anchors?

posted by janrinok on Wednesday March 27 2024, @08:12PM   Printer-friendly
from the I-didn't-know-that-... dept.

owl writes:

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?

Original Submission

 
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  • (Score: 2) by VLM on Thursday March 28 2024, @04:16PM (6 children)

    by VLM (445) Subscriber Badge on Thursday March 28 2024, @04:16PM (#1350711)

    Okay, but then why did Ken Thompson choose those characters?

    Speculation: Probably used an ASR-33

    Facts: Noobs think ^ is a shift-6 but on an ASR-33 it's control-n, so we're talking about shift 4 vs control n

    Speculation: Ken ran out of keys. This is not a 100+ key modern keyboard, so you're lucky it's shift-4 vs control-n and not even worse key combos.

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  • (Score: 2) by maxwell demon on Friday March 29 2024, @07:47AM (2 children)

    by maxwell demon (1608) on Friday March 29 2024, @07:47AM (#1350821) Journal

    For me, ^ doesn't even need shift, it's the key left of the 1 key.

    --
    The Tao of math: The numbers you can count are not the real numbers.

    • (Score: 2) by VLM on Friday March 29 2024, @07:39PM (1 child)

      by VLM (445) Subscriber Badge on Friday March 29 2024, @07:39PM (#1350900)

      it's the key left of the 1 key

      OK fine I give up what is your keyboard? Its not an AZERTY thing... You remap the key for some reason?

  • (Score: 2) by janrinok on Friday March 29 2024, @07:54PM (1 child)

    by janrinok (52) Subscriber Badge on Friday March 29 2024, @07:54PM (#1350903) Journal
    There is an image of the keyboard that Ken Thompson had available to him at the time. It has far fewer options than a modern keyboard.
    --
    [nostyle RIP 06 May 2025]

    • (Score: 2) by janrinok on Friday March 29 2024, @07:55PM

      by janrinok (52) Subscriber Badge on Friday March 29 2024, @07:55PM (#1350904) Journal
      The is an image in the source link ....
      --
      [nostyle RIP 06 May 2025]

  • (Score: 2) by janrinok on Friday March 29 2024, @08:02PM

    by janrinok (52) Subscriber Badge on Friday March 29 2024, @08:02PM (#1350906) Journal
    He had a Teletype Model 35. All this information is in the linked source. It had fewer keys that the ASR-33.
    --
    [nostyle RIP 06 May 2025]