POSIX "basic" regular expressions do not support + (nor ?!). Most implementations of sed add support for \+ but it's not a POSIX standard feature. If your goal is maximum portability you should avoid using it. Notice that you have to use \+ rather than the more common +.
echo "Hello World, I am an example-text" | sed 's#[^a-z0-9]\+#.#ig'
The -E flag enables "extended" regular expressions, which are a lot closer to the syntax used in Perl, JavaScript, and most other modern regex engines. With -E you don't need to have a backslash; it's simply +.
echo "Hello World, I am an example-text" | sed -E 's#[^a-z0-9]+#.#ig'
From https://www.regular-expressions.info/posix.html:
POSIX or "Portable Operating System Interface for uniX" is a collection of standards that define some of the functionality that a (UNIX) operating system should support. One of these standards defines two flavors of regular expressions. Commands involving regular expressions, such as grep and egrep, implement these flavors on POSIX-compliant UNIX systems. Several database systems also use POSIX regular expressions.
The Basic Regular Expressions or BRE flavor standardizes a flavor similar to the one used by the traditional UNIX grep command. This is pretty much the oldest regular expression flavor still in use today. One thing that sets this flavor apart is that most metacharacters require a backslash to give the metacharacter its flavor. Most other flavors, including POSIX ERE, use a backslash to suppress the meaning of metacharacters. Using a backslash to escape a character that is never a metacharacter is an error.
A BRE supports POSIX bracket expressions, which are similar to character classes in other regex flavors, with a few special features. Shorthands are not supported. Other features using the usual metacharacters are the dot to match any character except a line break, the caret and dollar to match the start and end of the string, and the star to repeat the token zero or more times. To match any of these characters literally, escape them with a backslash.
The other BRE metacharacters require a backslash to give them their special meaning. The reason is that the oldest versions of UNIX grep did not support these. The developers of grep wanted to keep it compatible with existing regular expressions, which may use these characters as literal characters. The BRE a{1,2} matches a{1,2} literally, while a\{1,2\} matches a or aa. Some implementations support \? and \+ as an alternative syntax to \{0,1\} and \{1,\}, but \? and \+ are not part of the POSIX standard. Tokens can be grouped with \( and \). Backreferences are the usual \1 through \9. Only up to 9 groups are permitted. E.g. \(ab\)\1 matches abab, while (ab)\1 is invalid since there's no capturing group corresponding to the backreference \1. Use \\1 to match \1 literally.
POSIX BRE does not support any other features. Even alternation is not supported.
(Emphasis mine.)
So my real question: is there any regex definition or implementation that does not treat either x+ or x\+ the same as xx*.
I can't think of any real world language or tool that supports neither + nor \+.
In the formal mathematical definition of regular expressions there are commonly only three operations defined:
- Concatenation: AB matches A followed by B.
- Alternation: A|B matches either A or B.
- Kleene star: R* matches 0 or more repetitions of R.
These three operations are enough to give the full expressive power of regular expressions†. Operators like ? and + are convenient in programming but not necessary in a mathematical context. If needed, they are defined in terms of the others: R? is R|ε and R+ is RR*.
† Mathematically speaking, that is. Features like back references and lookahead/lookbehind don't exist in formal language theory. Those features add additional expressive power not available in mathematical definitions of regular expressions.
