Raw Input

Desired Output

==Paragraph-1 Line 1===
==Paragraph-1 Line 2===
==Paragraph-1 Line 3===


==Paragraph-2 Line 1===
==Paragraph-2 Line 2===
==Paragraph-2 Line 3===

==Paragraph-3 Line 1===

==Paragraph-1 Line 1===
==Paragraph-2 Line 1===
==Paragraph-3 Line 1===

Script and Comments

Script₁ [ 1] /./!d [ 2] p [ 3] :loop [ 4] $d [ 5] N [ 6] /\n$/d [ 7] s/.*\n// [ 8] b loop	Comments 'Pattern space' and 'hold space' are abbreviated to 'PS' and 'HS', respectively. With Step [1], sed will delete blank lines and start a new cycle. When the first line of a paragraph is found, what we have to do is Print it (Step [2]). Skip the remaining lines of the paragraph . Steps [3] through [8] constitute a loop. This loop will Append next line if it exists (Step [4] and [5]). If the newly appended line is a blank one, '\n$' will match. This implies that the end of a paragraph is reached. We just discard the contents of PS and start a new cycle. Otherwise, we delete the first line of PS and start next iteration.
Script₂ [ 1] /./!d [ 2] p [ 3] :loop [ 4] $!N [ 5] s/.*\n$.$/\1/ [ 6] t loop [ 7] d	Comments This is a refined version of Script₁ Consider the following two cases: When the current line is the last one of the datafile, since command 'N' in Step [4] was not executed, PS will not contain a newline character. When the first blank line following a paragraph is appended, PS will NOT match '.\n.' since no character next to the newline exist. In these two cases, since no substitution will be made by Step [5], sed* will not perform the conditional branch in Step [6], but perform Step [7].
Script₃ [ 1] /^$/d [ 2] p [ 3] :loop [ 4] $!N [ 5] /\n./!d [ 6] g [ 7] b loop	Comments This script differs from Script₂ in Step [5] thru Step [7]. In Step [5], PS may contains Only one line, this implies that the end of file is reached since command 'N' in Step [4] was not performed. Two lines, where the second line is a blank one. This implies that the end of a paragraph is reached. Two lines, where the second line is not a blank one. Step [5] will discard lines of the first two cases. In Step [6], the content of PS is no longer needed, we use command 'g' to overwrite PS with the contents of HS, which is empty.