intact: pull the number out of an order line

one backslash gone: the same formula, still green

'Order' contains a d. There is something for the pattern to find, so there is nothing for Sheets to complain about.

intact: is this cell a whole number?

one backslash gone: is this cell a whole number?

intact: collapse repeated spaces

one backslash gone: collapse repeated spaces

REGEXREPLACE returns the original string when nothing matches. A function that does nothing is indistinguishable from a function that had nothing to do.

intact: does this look like money?

both backslashes gone: does this look like money?

only the escape on the dot survives

Backslashes do not vanish as a set. Whichever one a text pipeline eats is the one that decides the answer.

intact: is this row marked as a draft?

both backslashes gone: the parentheses become a capture group

Still a valid pattern. It now asks whether the cell is the word draft, which it is not.

intact: does the line end in a closing bracket?

one backslash gone: an unmatched bracket

This is the case we expect to fail loudly, and the reason the others are dangerous.

intact: extract a formatted total

every backslash gone: the dollar sign becomes an anchor

intact: reformat an ISO date as day/month/year

every backslash gone: the date comes back unreformatted

The output is still a date. It is still the date in the cell. A column of these looks like a column that has not been converted yet, not like a column that is broken.

intact: whole-word match

both backslashes gone: whole-word match

the other direction: a pipeline that escapes instead of eating

Some editors double a backslash rather than remove it. The pattern now looks for a literal backslash followed by a d.

a doubled backslash before a dot

the control: a pattern with no backslash at all is unharmed

Patterns that survive the trip are the ones that never needed a backslash. This is why simple examples in tutorials appear to work.