GREP help: Bitcoin address/key buried as text somewhere on hdd
[^13][a-km-zA-HJ-NP-Z1-9]{25,34}$.


...


  I removed the ^ and $ from the ends after when I realized that there is no reason to think it's at the beginning or end of a line ...


As you noted, the trailing dollar sign ($) anchors the search expression to an end-of-line (e.g. LF on Linux) - you probably do not want to do that.

However the [^13] does not anchor the search to start of line, inside square-brackets, the caret ^ means not. So [^13] means look for a string whose first 8-bit character is not a 1 nor a 3. It would match strings that begin with any other character - that is probably not what you want. If you want a string that begins with a 1 or a 3 just use [13]. If you wanted to anchor that to the start of a line you'd use ^[13] not [^13].


  ... (if I properly understand what that even means).


The expression ^[13][a-zA-Z0-9]{25,34} matches these lines:

1234567890123456789012345 is my secret code
3aklsdjlajkjvcnbxuxcjvhhkjhgtresds


where the 1 or 3 immediately follow a prior LF character (or are the first character in a file).

but does not match either of

my secret code is 1234567890123456789012345
    3aklsdjlajkjvcnbxuxcjvhhkjhgtresds