[SOLVED] grep "Size: [0-9]\+ MB"

iorih0304 · 08-23-2017, 05:03 AM

Hello everyone,
I study a command as follows:

Code:

dmidecode --type memory | grep "Size: [0-9]\+ MB"

I don't understand the meaning of " [0-9]\+ " .
What is the meaning of "\+" ?
Is it regular expression or ???
Thanks!

AwesomeMachine · 08-23-2017, 05:50 AM

Anything in square brackets means list or range. [0-9] means any single integer 0 thru 9 inclusive. [12345] means any combination of one or more in the list. I think the "\+" might produce no output. But this exercise is so mundane I can only hope it's a class assignment and not actual on-the-job stuff.

You might want to change the quotes from double to single.

iorih0304 · 08-23-2017, 05:56 AM

I actually know what the [0-9] means.
I just want to know what is \+ doing.

MadeInGermany · 08-23-2017, 06:33 AM

\+ is one of these questionable BRE extensions in Linux.
Clear and portable is \{1,\} or + in egrep (or grep -E that uses ERE).
It means the preceding character(-wildcard) must exist 1 or more times.

Code:

dmidecode --type memory | grep "Size: [0-9]\{1,\} MB"
dmidecode --type memory | egrep "Size: [0-9]+ MB"

iorih0304 · 08-23-2017, 10:14 PM

Quote:

Originally Posted by MadeInGermany

\+ is one of these questionable BRE extensions in Linux.
Clear and portable is \{1,\} or + in egrep (or grep -E that uses ERE).
It means the preceding character(-wildcard) must exist 1 or more times.

Code:

dmidecode --type memory | grep "Size: [0-9]\{1,\} MB"
dmidecode --type memory | egrep "Size: [0-9]+ MB"

Thank you.
The explanation is so understandable for me.

jlinkels · 08-23-2017, 10:22 PM

Quote:

Originally Posted by MadeInGermany

\+ is one of these questionable BRE extensions in Linux.
Clear and portable is \{1,\} or + in egrep (or grep -E that uses ERE).
It means the preceding character(-wildcard) must exist 1 or more times.

Code:

dmidecode --type memory | grep "Size: [0-9]\{1,\} MB"
dmidecode --type memory | egrep "Size: [0-9]+ MB"

I knew that as well [0-9]+ matches one or more [0-9]. Just like an '*' matches zero or more.
But why the '\' in front of \+? That would indicate a literal '+', wouldn't it?
I use [0-9]+ and [0-9]{3} quite often, but I was looking forward to an answer to [0-9]\+

jlinkels

pan64 · 08-24-2017, 03:22 AM

Quote:

Originally Posted by jlinkels

I was looking forward to an answer to [0-9]\+

jlinkels

You can always test:

Code:

root@host# dmidecode --type memory | egrep "Size: [0-9]\+ MB"
root@host# dmidecode --type memory | egrep "Size: [0-9]+ MB"
	Size: 8192 MB
	Size: 8192 MB
root@host# dmidecode --type memory | grep "Size: [0-9]\+ MB"
	Size: 8192 MB
	Size: 8192 MB
root@host# dmidecode --type memory | grep "Size: [0-9]+ MB"
root@host#

MadeInGermany · 08-24-2017, 06:52 AM

In ERE (extended regular expression) the special meaning is {m,n} and +
In BRE (basic regular expression) the special meaning is \{m,n\}
Someone thought it makes sense to have \+ in BRE following the same logic. This is in newer Linux only; it is not in the standards.
Also some take-overs from PRE (Perl regular expression, highly compatible with ERE) went into newer Linux BRE and ERE, like \b \s \d I wouldn't use them: no standard, not portable.
Instead use
\< \> (left and right word boundary, quite portable in BRE but also no standard)
[[:space:]]
[0-9] or [[:digit:]]

jlinkels · 08-24-2017, 08:36 PM

I knew there were some differences in syntax between BRE and ERE. I always thought ERE was an extension of BRE and upward compatible. Not so. That backslashes have an opposite effect in some situations in BRE and ERE I never knew. Never too old to learn.

jlinkels

pan64 · 08-25-2017, 01:43 AM

Quote:

Originally Posted by jlinkels

I knew there were some differences in syntax between BRE and ERE. I always thought ERE was an extension of BRE and upward compatible. Not so. That backslashes have an opposite effect in some situations in BRE and ERE I never knew. Never too old to learn.

jlinkels

Not so. Yes, you are right. And it is valid not only for grep, but for sed too.

AwesomeMachine · 08-25-2017, 03:02 AM

I thought in BRE 1\+1=2 is literal 1+1=2, and 1+1=2 is 111=2 or 1111=2, and so on.

edit: never mind. I performed pan64's recommended test from #7.