On 23.06.11 22:19, Lars Andersson wrote:
> Nice to have the file conversion integrated in AXIS.
> No need to change the axis name then.
Thank you for the nicely potted integration example.
I'll certainly keep it for reference.
[...]
> This is E2A that was run from outside of AXIS before
> #!/bin/bash
> sed -e 's/\( E\)\([0-9]*\)/ A\2/' $1
> Guru comments on this?
Now that's asking for opinion as well as help. ;-)
Here's 1.5c worth of one or the other: The above regex is in BRE (Basic
Regular Expression) form, and so is partly obscured by a flurry of backslashes.
Readability is improved (innit?) if we allow sed to use ERE (Extended Regular
Expressions), like this:
sed -re 's/( E)([0-9]*)/ A\2/' $1
Now it's human readable, yet has identical behaviour:
echo G1 X53.6223 Y37.9513 E49.6224 F2300 | sed -re 's/( E)([0-9]*)/ A\2/'
G1 X53.6223 Y37.9513 A49.6224 F2300
Now it's also easier to see that the regex may have a weakness, in that
[0-9]* matches "zero or more digits". That means it is no more selective
than:
sed -re 's/ E/ A/'
As we see here:
$ echo G1 X53.6223 Y37.9513 Eggnog F2300 | sed -re 's/( E)([0-9]*)/ A\2/'
G1 X53.6223 Y37.9513 Aggnog F2300
$ echo G1 X53.6223 Y37.9513 Eggnog 49.6224 F2300 | sed -re 's/ E/ A/'
G1 X53.6223 Y37.9513 Aggnog 49.6224 F2300
To make the second regex atom do anything, we need to ask for "one or
more", like so:
$ echo G1 X53.6223 Y37.9513 E49.6224 F2300 | sed -re 's/( E)([0-9]+)/ A\2/'
G1 X53.6223 Y37.9513 A49.6224 F2300
$ echo G1 X53.6223 Y37.9513 Eggnog F2300 | sed -re 's/( E)([0-9]+)/ A\2/'
G1 X53.6223 Y37.9513 Eggnog F2300
Now it only acts in the desired context. (Hopefully I haven't belaboured
the explanation too much, in trying to be explicit.)
Since sed, awk, grep, etc all understand ERE as well as BRE, I've never
found a downside to using the more readable form. Less typing too.
(Even vim can be weaned off its tedious backslash addiction by
prepending \v to the regex expression.)
There's also this advice in "man 7 regex":
»
Regular expressions ("RE"s), as defined in POSIX.2, come in two forms:
modern REs (roughly those of egrep; POSIX.2 calls these "extended" REs)
and obsolete REs (roughly those of ed(1); POSIX.2 "basic" REs).
Obsolete REs mostly exist for backward compatibility in some old
programs; they will be discussed at the end.
«
Whew! I'm glad we get to keep the good ones.
Erik
--
The wonderful thing about standards is that there are so many of them.
- Andy Tanenbaum
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