Pulling out a certain value from a string via regex

Question

I've see a bunch of examples, but I just can't seem to make this work. Can grep output only specified groupings that match? for example seems like it should work, but I get either errors or no output at all.

I want to do:

pathname="/a/long/path/of/mine/2x02 - bar.mp4"

All of the examples will be the long path, one or two digits, x and then 2 digits followed by a space, a - and a filename.

I want to parse this for the 02 value: https://regex101.com/ shows that \d{1,2}x(\d\d) should have match 1 = 02 in this example.

What I can't figure out is if I have

echo "$pathname" | sed -n 's/.*\d{1,2}x\(\d\d\)/1円/p'

or

echo $pathname | grep -oP '\d{1,2}x(\d\d)'

I get nothing. I can do:

echo $pathname | grep -oP '(\d\d)'

but there could be cases where there are other 2 digit values in a row, like if I had

/a/long/path/of/mine/12x02 - bar.mp4

in which case I don't think the above will specify the second match, so I prefer the more specific regex... IF I can use matching groups or something. I'm trying to do this in bash on Scientific Linux 7.1.

• alexis: sed and grep are old programs, and they don't accept "full" modern regular expressions. The details differ by OS, so see the manual pages on your system to see what they do accept. Use egrep (or grep -E, which is the same) to enable some of the regexp functionality.
• John1024: @alexis For GNU grep (which the OP appears to be using), the basic and extended (-E) regular expressions have "no difference in available functionality". Also, the OP is already using something more powerful than -E. He is using grep -P which means that his grep supports perl-style regexes.

Answer 1

As you were using grep with PCRE (-P), you can use this Regex pattern :

grep -Po '\d{1,2}x\K\d{2}(?= )' <<<"$pathname"

• \d{1,2}x will match one or two digits followed by x, then \K will discard the match

• \d{2} will match exactly two digits, the zero width positive lookahead pattern (?= ) ensures that we have a space after the two digits.

So this should fulfill your requirements.

Example :

$ grep -Po '\d{1,2}x\K\d{2}(?= )' <<<'/a/long/path/of/mine/2x02 - bar.mp4'
02
$ grep -Po '\d{1,2}x\K\d{2}(?= )' <<<'/a/long/path/of/mine/34x12 - bar.mp4'
12
$ grep -Po '\d{1,2}x\K\d{2}(?= )' <<<'/a/long/path/of/mine/0x1 - bar.mp4'
## No match
$ grep -Po '\d{1,2}x\K\d{2}(?= )' <<<'/a/long/path/of/mine/00x1 - bar.mp4'
## No match

• jmp242: I think part of my confusion was not knowing about doing <<< rather than feeding grep from echo | ... The other info about regex is very helpful also, so I'm marking this as the answer.

Answer 2

Using sed

With sed in basic mode, the braces need to be escaped:

$ echo "$pathname" | sed -n 's/.*[[:digit:]]\{1,2\}x\([[:digit:]][[:digit:]]\).*/1円/p'
02

For greater portability, I used [[:digit:]] in place of \d. I also added .* to the end so as to remove the trailing text.

Using grep -P

grep -P supports a look-behind feature but the look-behind text has to be of fixed length. So, we can look for a single digit preceding the x preceding the two digits that we want to display:

$ echo "$pathname" | grep -oP '(?<=\dx)(\d\d)'
02

Alternate path

Both of the above also work with the alternate path:

$ echo '/a/long/path/of/mine/12x02 - bar.mp4' | grep -oP '(?<=\dx)(\d\d)'
02
$ echo '/a/long/path/of/mine/12x02 - bar.mp4' | sed -n 's/.*[[:digit:]]\{1,2\}x\([[:digit:]][[:digit:]]\).*/1円/p'
02

Answer 3

Using just a posix shell

p=$pathname
p=${p##*/}
p=${p#*x}
p=${p%% *}
echo "$p"
#or on one line
p=${pathname##*/};p=${p#*x};p=${p%% *};echo "$p"

• jmp242: That was cool, but it just gives the last digit. I.e.: pathname="/a/long/path/of/mine/2x12 - bar.mp4" still gives me "2" rather than "12"
• fd0: @jmp242 I made an edit to meet your requirement.