Re: Pattern matching lines starting with double comment characters

[goncholden]

------- Original Message -------
On Sunday, February 26th, 2023 at 2:57 AM, Koichi Murase 
<myoga.murase@gmail.com> wrote:


> 2023年2月25日(土) 22:44 Tapani Tarvainen bash@tapanitarvainen.fi:
> 
> > What are you matching them with? Different tools have different
> > regular expressions.
> 
> 
> The variable name provided in the original question implies ERE, and
> ERE is unique (except one allows using implementation extensions).
> Although grep's ERE (where newlines mean alternation) and awk's ERE
> (where backslash escape is interpreted in bracket expressions) have
> additional rules, these differences do not seem to be relevant for the
> current question as far as I know.
> 
> > > But the pattern does not specify double comment characters
> > > 
> > > Case of ##
> > > Case of ;;
> > > Case of !!
> > > 
> > > How can adapt the pattern for these cases?
> 
> > [...]
> > pn_ere = "^[[:space:]]([#;!]{2,}|@c|//)[[:space:]]+"
> > [...]
> > pn_ere = "^[[:space:]]([#;!][#;!]+|@c|//)[[:space:]]+"
> 
> 
> The above two both match also #!, !#, #;, ;#, ;!, and !;, which do not
> seem to be requested.
> 
> > With egrep (grep -E) you could also do this:
> > 
> > pn_ere = "^[[:space:]]*(([#;!])\2+|@c|//)[[:space:]]+"
> 
> 
> \2 is undefined in ERE. The back references are only defined in BRE.
> It's an implementation extension and not always available with grep
> -E.
> 
> Also, `egrep' is not standardized, and now GNU grep produces warning messages 
> when one tries to use` egrep'. `grep -E' should be always
> used.
> 
> ----
> 
> The only portable way is to specify each just the same as you do for //:
> 
> pn_ere = "^[[:space:]]*(##|;;|!!|@c|//)[[:space:]]+"

This is good.