Plan9 sed - rationale for limitations


 
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Plan9 sed - rationale for limitations

Using the 9base port in {Net,Free}BSD, it appears that Plan 9's sed

1. restricts the max number of sed commands and thereby the length of sed scripts

2. does not recognise the \nn flag where nn is the number of a matched substring. See example below.

Questions:

1. Why would they limit the max number of sed commands?

2. Plan 9's regexp library is not the same as BSD's? (It's probably better hence my curiosity.) So the rules of re_format(7) do not apply? Is it not possible to match substrings, as below?

Code:
echo abcd|sed 's/\(.*b\)\(.*\)/\2\1/' 

FreeBSD sed: cdab 

Plan 9 sed:abcd

sed -f long_list_of_cmds.sed 

FreeBSD: (no problem) 

Plan 9:  "Too many commands"

Is it that the Plan 9 utils like sed and grep use ERE by default, instead of BRE? No BRE in Plan 9 utils?
# 2  
I haven't played with Plan9 in a long while, but, yes, I believe there is only a single flavor of regular expression in the Plan9 ecosystem (which is a nice respite from the many flavors in POSIX and real UNIX implementations, where each of ed, ex, grep, egrep, sed, and awk prescribe their own flavor).

I'm fairly sure a left-parens is already a metacharacter, in Plan9 (as with POSIX ERE), so backslash-escaping it (as in your example) actually treats it literally.

As to the "Too many commands" message, I have no clue. Perhaps it's a syntax error and not a script length limit?

Regards,
Alister
 

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