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Since sed does not cut it, let's just throw the towel and use Perl, at least it is LSB while grep GNU extensions are not :-)

• Print the entire matching part, no matching groups or lookbehind needed:

  cat <<EOS | perl -lane 'print m/\d+/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  12
  3456
  

• Single match per line, often structured data fields:

  cat <<EOS | perl -lape 's/.*?a(\d+).*/$1/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  1
  34
  

  With lookbehind:

  cat <<EOS | perl -lane 'print m/(?<=a)(\d+)/'
  a1 b2
  a34 b56
  EOS
  

• Multiple fields:

  cat <<EOS | perl -lape 's/.*?a(\d+).*?b(\d+).*/$1 $2/g'
  a1 c0 b2 c0
  a34 c0 b56 c0
  EOS
  

  Output:

  1 2
  34 56
  

• Multiple matches per line, often unstructured data:

  cat <<EOS | perl -lape 's/.*?a(\d+)|.*/$1 /g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1 
  34 78
  

  With lookbehind:

  cat EOS<< | perl -lane 'print m/(?<=a)(\d+)/g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1
  3478
  

Give up and use Perl

Since sed does not cut it, let's just throw the towel and use Perl, at least it is LSB while grep GNU extensions are not :-)

• Print the entire matching part, no matching groups or lookbehind needed:

  cat <<EOS | perl -lane 'print m/\d+/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  12
  3456
  

• Single match per line, often structured data fields:

  cat <<EOS | perl -lape 's/.*?a(\d+).*/$1/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  1
  34
  

  With lookbehind:

  cat <<EOS | perl -lane 'print m/(?<=a)(\d+)/'
  a1 b2
  a34 b56
  EOS
  

• Multiple fields:

  cat <<EOS | perl -lape 's/.*?a(\d+).*?b(\d+).*/$1 $2/g'
  a1 c0 b2 c0
  a34 c0 b56 c0
  EOS
  

  Output:

  1 2
  34 56
  

• Multiple matches per line, often unstructured data:

  cat <<EOS | perl -lape 's/.*?a(\d+)|.*/$1 /g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1 
  34 78
  

  With lookbehind:

  cat EOS<< | perl -lane 'print m/(?<=a)(\d+)/g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1
  3478
  

Since sed does not cut it, let's just throw the towel and use Perl, at least it is LSB while grep GNU extensions are not :-)

• Print the entire matching part, no matching groups or lookbehind needed:

  cat <<EOS | perl -lane 'print m/\d+/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  12
  3456
  

• Single match per line, often structured data fields:

  cat <<EOS | perl -lane 'print if s/.*?a(\d+).*/$1/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  1
  34
  

  With lookbehind:

  cat <<EOS | perl -lane 'print m/(?<=a)(\d+)/'
  a1 b2
  a34 b56
  EOS
  

• Multiple fields:

  cat <<EOS | perl -lane 'print if s/.*?a(\d+).*?b(\d+).*/$1 $2/g'
  a1 c0 b2 c0
  a34 c0 b56 c0
  EOS
  

  Output:

  1 2
  34 56
  

• Multiple matches per line, often unstructured data:

  cat <<EOS | perl -lane 'print if s/.*?a(\d+)|.*/$1 /g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1 
  34 78
  

  With lookbehind:

  cat EOS<< | perl -lane 'print m/(?<=a)(\d+)/g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1
  3478
  

Since sed does not cut it, let's just throw the towel and use Perl, at least it is LSB while grep GNU extensions are not :-)

• Print the entire matching part, no matching groups or lookbehind needed:

  cat <<EOS | perl -lane 'print m/\d+/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  12
  3456
  

• Single match per line, often structured data fields:

  cat <<EOS | perl -lape 's/.*?a(\d+).*/$1/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  1
  34
  

  With lookbehind:

  cat <<EOS | perl -lane 'print m/(?<=a)(\d+)/'
  a1 b2
  a34 b56
  EOS
  

• Multiple fields:

  cat <<EOS | perl -lape 's/.*?a(\d+).*?b(\d+).*/$1 $2/g'
  a1 c0 b2 c0
  a34 c0 b56 c0
  EOS
  

  Output:

  1 2
  34 56
  

• Multiple matches per line, often unstructured data:

  cat <<EOS | perl -lape 's/.*?a(\d+)|.*/$1 /g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1 
  34 78
  

  With lookbehind:

  cat EOS<< | perl -lane 'print m/(?<=a)(\d+)/g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1
  3478
  

Since sed does not cut it, let's just throw the towel and use Perl, at least it is LSB while grep GNU extensions are not :-)

• Print the entire matching part, no matching groups or lookbehind needed:

  printf 'a1 b2\na34 b56\n' | perl -lane 'print m/\d+/g'
  

  Output:

  12
  3456
  

• Single match per line, often structured data fields:

  printf 'a1 b2\na34 b56\n' | perl -lane 'print if s/.*?a(\d+).*/$1/g'
  

  Output:

  1
  34
  

  With lookbehind:

  printf 'a1 b2\na34 b56\n' | perl -lane 'print m/(?<=a)(\d+)/'
  

• Multiple fields:

  printf 'a1 c0 b2 c0\na34 c0 b56 c0\n' |\
    perl -lane 'print if s/.*?a(\d+).*?b(\d+).*/$1 $2/g'
  

  Output:

  1 2
  34 56
  

• Multiple matches per line, often unstructured data:

  printf 'a1 b2\na34 b56 a78 b90\n' | perl -lane 'print if s/.*?a(\d+)|.*/$1 /g'
  

  Output:

  1 
  34 78
  

  With lookbehind:

  printf 'a1 b2\na34 b56 a78 b90\n' | perl -lane 'print m/(?<=a)(\d+)/g'
  

  Output:

  1
  3478
  

Since sed does not cut it, let's just throw the towel and use Perl, at least it is LSB while grep GNU extensions are not :-)

• Print the entire matching part, no matching groups or lookbehind needed:

  cat <<EOS | perl -lane 'print m/\d+/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  12
  3456
  

• Single match per line, often structured data fields:

  cat <<EOS | perl -lane 'print if s/.*?a(\d+).*/$1/g'
  a1 b2
  a34 b56
  EOS
  

  Output:

  1
  34
  

  With lookbehind:

  cat <<EOS | perl -lane 'print m/(?<=a)(\d+)/'
  a1 b2
  a34 b56
  EOS
  

• Multiple fields:

  cat <<EOS | perl -lane 'print if s/.*?a(\d+).*?b(\d+).*/$1 $2/g'
  a1 c0 b2 c0
  a34 c0 b56 c0
  EOS
  

  Output:

  1 2
  34 56
  

• Multiple matches per line, often unstructured data:

  cat <<EOS | perl -lane 'print if s/.*?a(\d+)|.*/$1 /g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1 
  34 78
  

  With lookbehind:

  cat EOS<< | perl -lane 'print m/(?<=a)(\d+)/g'
  a1 b2
  a34 b56 a78 b90
  EOS
  

  Output:

  1
  3478