The author is circling around, but not quite reaching, a statement that POSIX Basic Regular Expressions work everywhere, with the caveat that that not everyone has caught up with version 8 of the Single Unix Specification, which has slightly changed BREs.
I've always been a stickler for being specific about which regex language your thing accepts, and whether it is to match any substring, or a prefix, or a suffix, or the whole thing, or a line, or a substring of a line, or whatever.
Here are some of the [more popular][1] ones, and then there are PCRE and Python.
It took me a while to learn that some of the older ones you see in e.g. grep are [specified by POSIX][2].
It drives me nuts when a developer documents something or other as being a "regex" but doesn't mention which dialect of regulation expression he's talking about. This habit is particularly common in the Rust, JavaScript, and Python communities, which seem to forget that their language's regular expression language isn't universal.
These already do not work in many tools which require those special characters to be escaped to have any meaning. An easy example is GNU grep, sed, etc. which use BRE ("Basic Regular Expressions") by default. The article mentions GNU coreutils but does not explain that `-E` is required to fix that behavior.