How to use sed/grep to extract text between two words?

GNU grep can also support positive & negative look-ahead & look-back: For your case, the command would be:

echo "Here is a string" | grep -o -P '(?<=Here).*(?=string)'

If there are multiple occurrences of Here and string, you can choose whether you want to match from the first Here and last string or match them individually. In terms of regex, it is called as greedy match (first case) or non-greedy match (second case)

$ echo 'Here is a string, and Here is another string.' | grep -oP '(?<=Here).*(?=string)' # Greedy match
 is a string, and Here is another 
$ echo 'Here is a string, and Here is another string.' | grep -oP '(?<=Here).*?(?=string)' # Non-greedy match (Notice the '?' after '*' in .*)
 is a 
 is another 

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sed -e 's/Here\(.*\)String/\1/'

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The accepted answer does not remove text that could be before Here or after String. This will:

sed -e 's/.*Here\(.*\)String.*/\1/'

The main difference is the addition of .* immediately before Here and after String.

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You can strip strings in Bash alone:

$ foo="Here is a String"
$ foo=${foo##*Here }
$ echo "$foo"
is a String
$ foo=${foo%% String*}
$ echo "$foo"
is a
$

And if you have a GNU grep that includes PCRE, you can use a zero-width assertion:

$ echo "Here is a String" | grep -Po '(?<=(Here )).*(?= String)'
is a

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If you have a long file with many multi-line ocurrences, it is useful to first print number lines:

cat -n file | sed -n '/Here/,/String/p'

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Through GNU awk,

$ echo "Here is a string" | awk -v FS="(Here|string)" '{print $2}'
 is a 

grep with -P(perl-regexp) parameter supports \K, which helps in discarding the previously matched characters. In our case , the previously matched string was Here so it got discarded from the final output.

$ echo "Here is a string" | grep -oP 'Here\K.*(?=string)'
 is a 
$ echo "Here is a string" | grep -oP 'Here\K(?:(?!string).)*'
 is a 

If you want the output to be is a then you could try the below,

$ echo "Here is a string" | grep -oP 'Here\s*\K.*(?=\s+string)'
is a
$ echo "Here is a string" | grep -oP 'Here\s*\K(?:(?!\s+string).)*'
is a

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You can use two s commands

$ echo "Here is a String" | sed 's/.*Here//; s/String.*//'
 is a 

Also works

$ echo "Here is a StringHere is a String" | sed 's/.*Here//; s/String.*//'
 is a

$ echo "Here is a StringHere is a StringHere is a StringHere is a String" | sed 's/.*Here//; s/String.*//'
 is a 

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This might work for you (GNU sed):

sed '/Here/!d;s//&\n/;s/.*\n//;:a;/String/bb;$!{n;ba};:b;s//\n&/;P;D' file 

This presents each representation of text between two markers (in this instance Here and String) on a newline and preserves newlines within the text.

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To understand sed command, we have to build it step by step.

Here is your original text

user@linux:~$ echo "Here is a String"
Here is a String
user@linux:~$ 

Let's try to remove Here string with substition option in sed

user@linux:~$ echo "Here is a String" | sed 's/Here //'
is a String
user@linux:~$ 

At this point, I believe you would be able to remove String as well

user@linux:~$ echo "Here is a String" | sed 's/String//'
Here is a
user@linux:~$ 

But this is not your desired output.

To combine two sed commands, use -e option

user@linux:~$ echo "Here is a String" | sed -e 's/Here //' -e 's/String//'
is a
user@linux:~$ 

Hope this helps

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All the above solutions have deficiencies where the last search string is repeated elsewhere in the string. I found it best to write a bash function.

    function str_str {
      local str
      str="${1#*${2}}"
      str="${str%%$3*}"
      echo -n "$str"
    }

    # test it ...
    mystr="this is a string"
    str_str "$mystr" "this " " string"

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You can use \1 (refer to http://www.grymoire.com/Unix/Sed.html#uh-4):

echo "Hello is a String" | sed 's/Hello\(.*\)String/\1/g'

The contents that is inside the brackets will be stored as \1.

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Problem. My stored Claws Mail messages are wrapped as follows, and I am trying to extract the Subject lines:

Subject: [SLC38A9 lysosomal arginine sensor; mTORC1 pathway] Key molecular
 link in major cell growth pathway: Findings point to new potential
 therapeutic target in pancreatic cancer [mTORC1 Activator SLC38A9 Is
 Required to Efflux Essential Amino Acids from Lysosomes and Use Protein as
 a Nutrient] [Re: Nutrient sensor in key growth-regulating metabolic pathway
 identified [Lysosomal amino acid transporter SLC38A9 signals arginine
 sufficiency to mTORC1]]
Message-ID: <20171019190902.18741771@VictoriasJourney.com>

Per A2 in this thread, How to use sed/grep to extract text between two words? the first expression, below, "works" as long as the matched text does not contain a newline:

grep -o -P '(?<=Subject: ).*(?=molecular)' corpus/01

[SLC38A9 lysosomal arginine sensor; mTORC1 pathway] Key

However, despite trying numerous variants (.+?; /s; ...), I could not get these to work:

grep -o -P '(?<=Subject: ).*(?=link)' corpus/01
grep -o -P '(?<=Subject: ).*(?=therapeutic)' corpus/01
etc.

Solution 1.

Per Extract text between two strings on different lines

sed -n '/Subject: /{:a;N;/Message-ID:/!ba; s/\n/ /g; s/\s\s*/ /g; s/.*Subject: \|Message-ID:.*//g;p}' corpus/01

which gives

[SLC38A9 lysosomal arginine sensor; mTORC1 pathway] Key molecular link in major cell growth pathway: Findings point to new potential therapeutic target in pancreatic cancer [mTORC1 Activator SLC38A9 Is Required to Efflux Essential Amino Acids from Lysosomes and Use Protein as a Nutrient] [Re: Nutrient sensor in key growth-regulating metabolic pathway identified [Lysosomal amino acid transporter SLC38A9 signals arginine sufficiency to mTORC1]]                              

Solution 2.*

Per How can I replace a newline (\n) using sed?

sed ':a;N;$!ba;s/\n/ /g' corpus/01

will replace newlines with a space.

Chaining that with A2 in How to use sed/grep to extract text between two words?, we get:

sed ':a;N;$!ba;s/\n/ /g' corpus/01 | grep -o -P '(?<=Subject: ).*(?=Message-ID:)'

which gives

[SLC38A9 lysosomal arginine sensor; mTORC1 pathway] Key molecular  link in major cell growth pathway: Findings point to new potential  therapeutic target in pancreatic cancer [mTORC1 Activator SLC38A9 Is  Required to Efflux Essential Amino Acids from Lysosomes and Use Protein as  a Nutrient] [Re: Nutrient sensor in key growth-regulating metabolic pathway  identified [Lysosomal amino acid transporter SLC38A9 signals arginine  sufficiency to mTORC1]] 

This variant removes double spaces:

sed ':a;N;$!ba;s/\n/ /g; s/\s\s*/ /g' corpus/01 | grep -o -P '(?<=Subject: ).*(?=Message-ID:)'

giving

[SLC38A9 lysosomal arginine sensor; mTORC1 pathway] Key molecular link in major cell growth pathway: Findings point to new potential therapeutic target in pancreatic cancer [mTORC1 Activator SLC38A9 Is Required to Efflux Essential Amino Acids from Lysosomes and Use Protein as a Nutrient] [Re: Nutrient sensor in key growth-regulating metabolic pathway identified [Lysosomal amino acid transporter SLC38A9 signals arginine sufficiency to mTORC1]]

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ripgrep

Here is the example using rg:

$ echo Here is a String | rg 'Here\s(.*)\sString' -r '$1'
is a

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