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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: 4, Informative) by Geoff Clare on Friday March 29 2024, @11:03AM

    by Geoff Clare (2397) on Friday March 29 2024, @11:03AM (#1350832)

    They are actually slightly different. If the line is indented, 0 goes to the very beginning but ^ goes to the first character after the indent.

    Starting Score:    1  point
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       Informative=3, Total=3
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    Total Score:   4