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Word wrap: more adventures with sed

When I first pasted the transcript of C. Scott’s talk, it was ugly and unwrapped and splurted off the right edge of my screen. So I decided to learn more regular expressions. This is what happened.

Attempt #1: Mrah. why does cat foo.txt | sed ’s/(.{1,80}( +|$\n?)|(.{1,80})/\1\3\n/g’ not work? \3 is not an invalid reference, you silly thing.

D’oh moment #1: Oh. escaping { and ( with \ is needed with sed. now…

Attempt #2: why doesn’t cat foo.txt | sed ’s/\(.\{1,80\}\)\( +|$\n?\)|\(.\{1,80\}\)/\1\3\n/g’ work?

Attempt #3 (gave up): Ok, simplifying… cat foo.txt | sed ’s/\(.\{1,80\}\)/\1\n/g’ will work, but not split across word boundaries when possible. Good enough for me (for now).

<br />#!/bin/sh<br /><br />if [ $# -ne 1 ]; then<br />   echo Usage: wrap file.txt<br />   exit 127<br />fi<br /><br />cat $1 | sed 's/\(.\{1,80\}\)/\1\n/g'<br />exit 0<br />

Translation from bahasa geek, for parents and other noncoders: I was trying to come up with a way to quickly wrap long lines of text (the equivalent of hitting Enter to put a newline in the middle of long sentences so that they’ll fit onto a page).

Sed is a stream editor that I used to do this, and I wrote a shell script (#!/bin/sh) that fed (cat) out the file I gave the script ($1 – which is a variable that I’d replace with the file I wanted to wrap, when the script was actually run). This replaces (s/) instances of an 80-character group, \(.\{1,80\}\) – the escaped parentheses, \( and \), say this is a group of characters, the period says “any non-whitespace character,” and the “1, 80″ inside the escaped brackets says “up to 80 of them all together.”

It replaces it with that same 80-character group (using \1, which is a backreference – basically, “the first group that you talked about here,” which is what we were doing with the parentheses – and then a newline, \n, which is like the Enter key. Basically, this script has the same effect as me going through a text document the following way:

1. hit the right arrow key 80 times
2. hit Enter
3. repeat steps 1 and 2 until you reach the end of the document

It’s a dumb script because it’ll break in the middle of words but I couldn’t figure out how to make it pay attention to word boundaries. I tried – see above attempts – but gave up because I didn’t have time to chase the answer down. I’m hoping that somebody reading this might be able to spot where I went wrong. Halp?