r - How do I deal with special characters like ^$.?*|+()[{ in my regex?

Question

I want to match a regular expression special character, ^$.?*|+()[{. I tried:

x <- "a[b"
grepl("[", x)
## Error: invalid regular expression '[', reason 'Missing ']''

(Equivalently stringr::str_detect(x, "[") or stringi::stri_detect_regex(x, "[").)

Doubling the value to escape it doesn't work:

grepl("[[", x)
## Error: invalid regular expression '[[', reason 'Missing ']''

Neither does using a backslash:

grepl("[", x)
## Error: '[' is an unrecognized escape in character string starting ""["

How do I match special characters?

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Some special cases of this in questions that are old and well written enough for it to be cheeky to close as duplicates of this:
Escaped Periods In R Regular Expressions
How to escape a question mark in R?
escaping pipe ("|") in a regex