regular expressions memory

wakatana · 07-25-2009, 10:52 AM

Hi all, i have read in one site (i can put link, but is in Czech language

) that regular expressions have some kind of memory. If i understood it propertly when i want to remebmer some part of regexp i just put it between \( and ) and then call it by \1 \2 \3 etc. where \$number is number of memored expression. So i tried some examples bud didnt work for me

:

lets say i want to know only UID and HOME from /etc/passwd so i tried followings:

Code:

grep -E "^[^:]*:[^:]*:\([^:]*):[^:]*:[^:]*:\([^:]*):[^:]*" /etc/passwd
grep -E "^[^:]*:[^:]*:\([^:])*:[^:]*:[^:]*:\([^:])*:[^:]*" /etc/passwd

but neither one works. I know i didnt call \1 and \2 but i dont know how.

also tried append "end" string to end of all lines of file

Code:

sed -e 's/\(.*)/\1end/' file_name

with the same result as prevouis.

I know both tasks can be done using some other utils such as sed. But just for curiosity i want know more about regexp memory. Thanks a lot

colucix · 07-25-2009, 11:09 AM

1) The syntax to remember part of the regexp is $...$, that is you have to escape both the parentheses. I don't know how to use it with grep anyway, or even if it's possible. I thought it was a sed feature.

2) to append a string at the end of each lines using sed:

Code:

sed 's/$/end/g' file

3) to extract info from the /etc/passwd file you can try awk using a colon as field separator:

Code:

awk -F: '{print $6}' /etc/passwd

wakatana · 07-25-2009, 11:21 AM

Tried it with $...$ in sed, so following command works same as previous

Code:

sed -e 's/\(.*\)/\1end/g' filename

thanks

in grep, now accept command by bash, but still dont know how to prnt only \1 and \2. Maybe youre right and in grep is not possible

Code:

grep "^[^:]*:\([^:]*\):[^:]*:[^:]*:[^:]*:\([^:]*\):[^:]*" passwd

PS: yes i know awk can do that, but i want learn more about this feature. tkanks anyway

colucix · 07-25-2009, 12:41 PM

Quote:

Originally Posted by wakatana

in grep, now accept command by bash, but still dont know how to prnt only \1 and \2. Maybe youre right and in grep is not possible

grep has the -o option to print only the part of a line that matches the regular expression. An example of "regexp memory" usage not strictly related to sed is here.

wakatana · 07-26-2009, 08:42 AM

Thanks for reply, I understand that, with sed i can do exactly what i was looking for, just enclose part of regexp into $...$ and and then "call" it by \number. BTW grep -o is also interesting, thanks

chrism01 · 07-27-2009, 01:03 AM

Here's a good sed manual http://www.grymoire.com/Unix/Sed.html
Regexes http://www.grymoire.com/Unix/Regular.html

wakatana · 10-13-2009, 08:54 AM

thanks

, sorry for long response