Characters that Need an Escape

[Doc Trins O'Grace]

I was looking at the awk manual, but I didn't really see an answer to my question. Perhaps one of you more knowledgeable folks know:

What is the exhaustive list of characters that require an escape to use them as constants in a regular expression?

Thank you for your time and attention.

Have a very good good Friday!

[Ed Morton]

Assuming you mean outside of a bracket expression in a current POSIX spec
Extended Regular Expression:

^ $ . * + ( ) [ ? | \ { }

The caveat being that `{` and `}` only NEED to be escaped if they look like an
RE interval specification, `{<digits>}`, otherwise they're already literal.

Regards,

Ed.

[Geoff Clare]

That last part is only true for some versions of awk. POSIX says the
results are undefined, and some awks report such uses as an error:

Solaris:

$ echo foo | awk '/foo{/'
awk: /foo{/: syntax error Context is:
>>> /foo{/ <<<
$ echo foo | awk '/foo{x}/'
awk: /foo{x}/: syntax error Context is:
>>> /foo{x}/ <<<

HP-UX:

$ echo foo | awk '/foo{/'
awk: There is a regular expression error.
{} imbalance.
The source line number is 1.
The error context is
>>> /foo{/ <<<
$ echo foo | awk '/foo{x}/'
awk: There is a regular expression error.
Invalid {} repetition.
The source line number is 1.
The error context is
>>> /foo{x}/ <<<

--
Geoff Clare <net...@gclare.org.uk>

[Doc Trins O'Grace]

Thank you, Ed and Geoff.

I guess I should have mentioned I am using Awk 3.1.1.

An old programmer using an old interpreter.

[Ed Morton]

GNU awk 3.1.1? The only difference between that and a current awk (get a new one
- seriously!!!) are that RE intervals (e.g. `{3}`) would need to be enabled
using `--re-interval` or `--posix` in that old version so `{` and `}` are
strictly literal otherwise.

Ed.