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Difference between space and [[:space:]] in regular expression

 
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# 1  
Old 07-25-2013
Difference between space and [[:space:]] in regular expression
May I know the difference between space in keyboard and [[:space:]] in regular expression

I entered the following
Code:
find . -type f -print | xargs grep -n 'dt=' | cut -d":" -f3 | sed 's/^ *dt=/dt=/g'
After "^" there is a space.
and the result is...
dt=`date +%Y%m%d%H%M%S`
        dt=`date +%Y%m%d`
dt=`date +%Y%m%d`
        dt=`date +%Y%m%d%H%M%S`
        dt=`date +%Y%m%d%H%M%S`
dt=`date +%Y%m%d%H%M%S`
        dt=`date +%Y%m%d%H%M%S`
        dt=`date +%Y%m%d%H%M%S`
        dt=`date +%Y%m%d%H%M%S`
dt=`date +%Y%m%d%H%M%S`
        dt=`date +%Y%m%d%H%M%S`
        dt=`date +%Y%m%d%H%M%S`

and when i used [[:space:]]

Code:
find . -type f -print | xargs grep -n 'dt=' | cut -d":" -f3 | sed 's/^[[:space:]]*dt=/dt=/g'
and the I got the expected result
dt=`date +%d%m%y_%H%M%S`
dt=`date +%d%m%y_%H%M%S`
dt=`date +%d%m%y_%H%M%S`
dt=`date +%d%m%y_%H%M%S`

---------- Post updated at 11:35 AM ---------- Previous update was at 11:31 AM ----------

By including space it did trimmed spaces for few of them.
# 2  
Old 07-25-2013
[[:space:]] means any whitespace, so it matches tabs as well as spaces.
These 2 Users Gave Thanks to Corona688 For This Post:
bobbygsk RavinderSingh13
# 3  
Old 07-25-2013
Your sed is redundant by the way,
Code:
sed 's/^[[:space:]]*//g'

is all that's needed.

But whenever you find yourself doing stuff | cut | sed | grep | kitchen | sink, you might as well roll it up into one simple awk.

Code:
find . -type f -print | xargs awk -F: '/dt=/ { sub(/^[ \t]*/, "", $3); print $3 }'

# 4  
Old 07-25-2013
Actually character class [:blank:] matches spaces and tabs.

Character class [:space:] matches spaces and tabs as well as newline, vertical tab, form feed, carriage return.

For further reference:

Character Classes and Bracket Expressions

POSIX Bracket Expressions
These 2 Users Gave Thanks to Yoda For This Post:
bobbygsk RavinderSingh13
# 5  
Old 07-25-2013
Quote:
Originally Posted by Yoda
Actually character class [:blank:] matches spaces and tabs.

Character class [:space:] matches spaces and tabs as well as newline, vertical tab, form feed, carriage return.
Which makes little difference in a usual regex, being that sed removes newlines before doing regex matching..
This User Gave Thanks to Corona688 For This Post:
bobbygsk
# 6  
Old 07-25-2013
I always feel comfortable doing in bits as I do not understand awk quite well. I only use awk for printing or for simpler awk code. So I use combination of all grep, cut, etc...
Thanks for providing in awk.
Code:
find . -type f -print | xargs awk -F: '/dt=/ { sub(/^[ \t]*/, "", $3); print $3 }'

Got the following result
Code:
awk: Cannot find or open file nn.

 The input line number is 1. The file is nn.
 The source line number is 1.













awk: Cannot find or open file ./XX/XXXXXXXXXX.

 The input line number is 113. The file is ./XX/XXXXXXXX.
 The source line number is 1.

There is no filename with nn in my unix folders.
However if I use my code above posts, i get good results
# 7  
Old 07-25-2013
No matter what I do I cannot induce awk to do that, so I figure you have some very strange filenames fed into it, possibly ones with spaces or =. Or perhaps a typo in the awk section.

cut | sed | grep | kitchen | sink code is slow, is the thing. And half the threads we get here are 'why is my cat | grep | sed | kitchen | sink so slow', so I think it often does matter.
 
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