Re: Adding new escapes to regex module

On 2022-08-16 22:14, Barry Scott wrote:
> > On 16 Aug 2022, at 21:24, MRAB <[email protected]> wrote:
> > 
> > Other regex implementations have escape sequences for horizontal whitespace (`\h` and `\H`) and vertical whitespace (`\v` and `\V`).
> > 
> > The regex module already supports `\h`, but I can't use `\v` because it represents `\0x0b', as it does in the re module.
>
> You seem to be mixing the use \ as the escape for strings and the \ that re uses.
> Is it the behaviour that '\<unknown>' becomes '\\<unknown>' that means this is a breaking change?
>
> Won't this work?
> ```
> re.compile('\v:\\v')
> # which is the same as
> re.compile(r'\x0b:\v')
> ```
>
Some languages, e.g. Perl, have a dedicated syntax for writing regexes, 
and they take `\n` (a backslash followed by 'n') to mean "match a newline".

Other languages, including Python, use string literals and can contain 
an actual newline, but they also take `\n` (a backslash followed by 'n') 
to mean "match a newline".

Thus:

 >>> print(re.match('\n', '\n')) # Literal newline.
<re.Match object; span=(0, 1), match='\n'>
 >>> print(re.match('\\n', '\n')) # `\n` sequence.
<re.Match object; span=(0, 1), match='\n'>

On the other hand:

 >>> print(re.match('\b', '\b')) # Literal backspace.
<re.Match object; span=(0, 1), match='\x08'>
 >>> print(re.match('\\b', '\b')) # `\b` sequence, which means a word 
boundary.
None
 >>>

The problem is that the re and regex modules already have the `\v` (a 
backslash followed by 'v') sequence to mean "match the '\v' character", so:

re.compile('\v')

and:

re.compile('\\v')

mean exactly the same.

> > Now that someone has asked for it, I'm trying to find a nice way of adding it, and I'm currently thinking that maybe I could use `\y` and `\Y` instead as they look a little like `\v` and `\V`, and, also, vertical whitespace is sort-of in the y-direction.
> > 
> > As far as I can tell, only ProgressSQL uses them, and, even then, it's for what everyone else writes as `\b` and `\B`.
> > 
> > I want the regex module to remain compatible with the re module, in case they get added there sometime in the future.
> > 
> > Opinions?