Using BRE and ERE in awk How does `awk` distinguish between use of `BRE` versus `ERE` ? For `sed` there is the `-E` option one can use.

It does not need to distinguish between the two since regular expressions in `awk` are always POSIX _extended_ regular expressions.

From the POSIX specification for `awk`:

> The awk utility shall make use of the extended regular expression notation [...]

I'm not aware of any `awk` implementation that introduces a non-standard option for switching to POSIX _basic_ regular expressions.

The `sed` utility always uses POSIX _basic_ regular expressions unless you use it with its (so far) non-standard but fairly commonly available `-E` option. The `grep` utility also defaults to POSIX _basic_ regular expressions, but the `-E` option that enables it to use POSIX _extended_ regular expressions is actually standard (while `-P` for Perl-compatible regular expressions, PCRE, is non-standard and not commonly implemented on non-GNU systems). The `grep` utility can additionally interpret the given expressions as plain strings with its standard `-F` option.