Add text between multiple lines with sed if previous line does not match pattern

Question

I'm struggling to add <dl> tags around a definition list with sed, and perhaps there is an easier way to do it (I'd love to know).

I would like to search a file for any lines that contain a <dt>, but only match if the previous line does not contain <dt> or <dd>. When a match is found, insert a <dl>.

My attempt so far (which doesn't match at all):

sed '/^((?!<dt>).)*$/ {
    N
    /<dt>/ {
        s/<dt>/<dl><dt>/
    } 
}' file

And the file

# TODO #

 * Set up mail transfer agent
 * Reconfigure timezone

```bash
dpkg-reconfigure tzdata
```

# Hardware #
  <dt>RAM</dt>
  <dd>2GB</dd>

# Partitions #

<dt>`/dev/sda1`</dt>
<dd>/boot</dd>
<dt>`/dev/sda2`</dt>
<dd>/</dd>

The purpose of all of this is to write a parser that converts moinmoin wiki markup to markdown, for porting to a new wiki engine. The defintion list is currently made with the following rule:

sed -i 's/^ \(.*\):: \(.*\)$/  <dt>\1<\/dt>\n  <dd>\2<\/dd>/' file

I would like the output to look like this:

# TODO #

 * Set up mail transfer agent
 * Reconfigure timezone

```bash
dpkg-reconfigure tzdata
```

# Hardware #
<dl>
  <dt>RAM</dt>
  <dd>2GB</dd>
</dl>

# Partitions #

<dl>
<dt>`/dev/sda1`</dt>
<dd>/boot</dd>
<dt>`/dev/sda2`</dt>
<dd>/</dd>
</dl>

Note that I'd like valid html as much as possible. There needs to be a closing tag for each opening tag.

Answer 1

This may be more sed than you expect, but I think this is the best way to accomplish what you want with sed.

This script:

• Inserts a line <dl> before each line that contains a <dt>, if no other <dt>, <dd>, or <dl> preceded the line.

• Appends a line </dl> after each line containing <dd>, if the line after the line containing <dd> does not contain <dd>.

This sed script uses the sed hold space to remember the previous line so it can be checked for the <d[tdl]> tags before inserting the <dl> tag. It also uses relative addressing ADDR,+N to allow adding the closing </dl> tag. A special case is required to detect if a <dd> is on the last line of the file and needs an appended </dl>. Testing (t and T) and branching (b) are used extensively to implement the logic.

#!/bin/sh

sed '
    /<dt>/ {
        x                  # exchange pattern and hold space
        s/<d[tdl]>//       # subsitutue, just testing for pattern
        g                  # copy hold space back, overwriting pattern space
        t end              # branch to :end if previous subsitution successful
        i \
<dl>
    }

    $ {
        /<dd>/ ! b end     # if <dd> on last line, append </dl>
        a \
</dl>
    }

    /<dd>/,+1 {            # on each line containing <dd> and the line after
        /<dd>/ b end       # if does not contain <dd>, insert </dl>
        i \
</dl>
    }

:end
    h                      # copy pattern space to hold space for next round

' "$@"

This script modifies the sample data to look like this:

[...]

# Hardware #
<dl>
  <dt>RAM</dt>
  <dd>2GB</dd>
</dl>

# Partitions #

<dl>
<dt>`/dev/sda1`</dt>
<dd>/boot</dd>
<dt>`/dev/sda2`</dt>
<dd>/</dd>
</dl>