docs: move extensions and incompatibilities to docs/src/extensions.md

Start an mdBook under docs/, laid out as in coreutils, and move the list of
extensions and incompatibilities out of the README into it. The README keeps
a pointer.

A Documentation workflow builds the book with mdbook 0.5.0, the version the
uutils website uses, whenever docs/ changes, and uploads it as an artifact.
mdbook 0.5 rejects the `multilingual` field coreutils still carries (the
website strips it before building), so the book leaves it out.

.github/workflows/documentation.yml

@@ -0,0 +1,41 @@
+name: Documentation
+
+on:
+  pull_request:
+    paths:
+      - 'docs/**'
+      - '.github/workflows/documentation.yml'
+  push:
+    branches:
+      - main
+    paths:
+      - 'docs/**'
+      - '.github/workflows/documentation.yml'
+
+permissions:
+  contents: read
+
+jobs:
+  build-doc:
+    name: Build the mdBook documentation
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v7
+        with:
+          persist-credentials: false
+
+      - name: Install mdbook
+        uses: taiki-e/install-action@v2
+        with:
+          tool: mdbook@0.5.0
+
+      - name: Build documentation
+        run: mdbook build docs
+
+      - name: Upload documentation
+        uses: actions/upload-artifact@v7
+        with:
+          name: documentation
+          path: docs/book

README.md

@@ -88,73 +88,8 @@ cargo test
 ```
 
 ## Extensions and incompatibilities
-### Supported GNU extensions
-* Command-line arguments can be specified in long (`--`) form.
-* Spaces can precede a regular expression modifier.
-* `I` can be used in as a synonym for the `i` (case insensitive) substitution
-  flag.
-* `M` and `m` substitution flags allow multi-line matching.
-* In addition to `\n`, other escape sequences (octal, hex, C) are supported
-  in the strings of the `y` command.
-  Under POSIX these yield undefined behavior.
-* The `a`, `c`, and `i` commands do not require an initial backslash,
-  allow text to appear on the same line, and support escape sequences
-  in the specified text.
-* The `a`, `i`, `=`, `l`, `q` and `r` commands support address range as an extension to POSIX.
-* The substitution command replacement group `\0` is a synonym for &.
-* An `F` command outputs the name of the file currently being processed.
-* A `Q` command (optionally followed by an exit code) quits immediately.
-* The `q` command can be optionally followed by an exit code.
-* A `W` command writes to a file the pattern's first line.
-* An `R` command reads one line at a time from a file.
-* The `l` command can be optionally followed by the output width.
-* The `--follow-symlinks` option for in-place editing.
-* The `--sandbox` option that limits potentially destructive commands.
-* Address 0 can be used to specify an address range that is already
-  active on line 1 and can finish with the specified regular expression.
-* Address steps can be specified in the form of start~step and start,~step
-  ranges.
-* Address 0 can be used in the `r` command to prepend a file.
-
-### Supported BSD and GNU extensions
-* The second address in a range can be specified as a relative address with +N.
-* In-place editing of file with the `-i` flag.
-
-### New extensions
-* Unicode characters can be specified in regular expression pattern, replacement
-  and transliteration sequences using `\uXXXX` or `\UXXXXXXXX` sequences.
-
-### Incompatible extensions
-The `-U` or `--uutil-extensions` option enables useful extensions or bug fixes
-that aren't compatible with GNU sed or POSIX.
-
-* The `l` command lists Unicode characters using the `\uXXXX` and `\UXXXXXXXX`
-  escapes rather than as octal UTF-8 byte sequences.
-
-### Incompatibilities
-* Similarly to GNU _sed_, input is processed as raw bytes or as valid UTF-8
-  (this includes 7-bit ASCII) based on the locale as specified by the
-  `LC_ALL`, `LC_CTYPE`, and `LANG` environment variables,
-  with the default being byte processing.
-  However, in contrast with GNU _sed_, other locales (e.g. ISO-8859-1)
-  are not supported. If the input is in another code page or encoding
-  and requires locale-specific processing (e.g. ignore/map case,
-  character classes), consider converting it through UTF-8 to ensure
-  the correct handling of locale-specific regular expressions.
-  This _sed_ program can also handle arbitrary byte sequences
-  if no part of the input requires treating it as a Rust String.
-* Back-references aren't supported when input is processed as bytes
-  (`LC_ALL=C`).
-* The command will report an error and fail if duplicate labels are found
-  in the script.
-  This matches the BSD behavior. The GNU version accepts duplicate labels.
-* The last line (`$`) address is interpreted as the last non-empty line of
-  the last file.  If files specified in subsequent arguments until the last
-  one are empty, then the last line condition will never be triggered.
-  This behavior is consistent with the
-  [original implementation](https://github.com/dspinellis/unix-history-repo/blob/Research-V7/usr/src/cmd/sed/sed1.c#L665).
-* Labels are parsed for alphanumeric characters. The BSD version parses them
-  until the end of the line, preventing ; to be used as a separator.
+The GNU, BSD and new extensions _sed_ supports, and where it differs from GNU
+_sed_, are listed in [docs/src/extensions.md](docs/src/extensions.md).
 
 ## GNU test suite compatibility
 

docs/.gitignore

@@ -0,0 +1 @@
+book

docs/book.toml

@@ -0,0 +1,7 @@
+[book]
+language = "en"
+src = "src"
+title = "uutils sed Documentation"
+
+[output.html]
+git-repository-url = "https://github.com/uutils/sed/tree/main/docs/src"

docs/src/SUMMARY.md

@@ -0,0 +1,5 @@
+# Summary
+
+[Introduction](index.md)
+
+* [Extensions and incompatibilities](extensions.md)

docs/src/extensions.md

@@ -0,0 +1,73 @@
+# Extensions and incompatibilities
+
+The main goal of the project is compatibility with GNU _sed_, but _sed_ also
+supports features that GNU _sed_ does not, and differs from it in a few places.
+Below is a list of these extensions and incompatibilities.
+
+## Supported GNU extensions
+* Command-line arguments can be specified in long (`--`) form.
+* Spaces can precede a regular expression modifier.
+* `I` can be used in as a synonym for the `i` (case insensitive) substitution
+  flag.
+* `M` and `m` substitution flags allow multi-line matching.
+* In addition to `\n`, other escape sequences (octal, hex, C) are supported
+  in the strings of the `y` command.
+  Under POSIX these yield undefined behavior.
+* The `a`, `c`, and `i` commands do not require an initial backslash,
+  allow text to appear on the same line, and support escape sequences
+  in the specified text.
+* The `a`, `i`, `=`, `l`, `q` and `r` commands support address range as an extension to POSIX.
+* The substitution command replacement group `\0` is a synonym for &.
+* An `F` command outputs the name of the file currently being processed.
+* A `Q` command (optionally followed by an exit code) quits immediately.
+* The `q` command can be optionally followed by an exit code.
+* A `W` command writes to a file the pattern's first line.
+* An `R` command reads one line at a time from a file.
+* The `l` command can be optionally followed by the output width.
+* The `--follow-symlinks` option for in-place editing.
+* The `--sandbox` option that limits potentially destructive commands.
+* Address 0 can be used to specify an address range that is already
+  active on line 1 and can finish with the specified regular expression.
+* Address steps can be specified in the form of start~step and start,~step
+  ranges.
+* Address 0 can be used in the `r` command to prepend a file.
+
+## Supported BSD and GNU extensions
+* The second address in a range can be specified as a relative address with +N.
+* In-place editing of file with the `-i` flag.
+
+## New extensions
+* Unicode characters can be specified in regular expression pattern, replacement
+  and transliteration sequences using `\uXXXX` or `\UXXXXXXXX` sequences.
+
+## Incompatible extensions
+The `-U` or `--uutil-extensions` option enables useful extensions or bug fixes
+that aren't compatible with GNU sed or POSIX.
+
+* The `l` command lists Unicode characters using the `\uXXXX` and `\UXXXXXXXX`
+  escapes rather than as octal UTF-8 byte sequences.
+
+## Incompatibilities
+* Similarly to GNU _sed_, input is processed as raw bytes or as valid UTF-8
+  (this includes 7-bit ASCII) based on the locale as specified by the
+  `LC_ALL`, `LC_CTYPE`, and `LANG` environment variables,
+  with the default being byte processing.
+  However, in contrast with GNU _sed_, other locales (e.g. ISO-8859-1)
+  are not supported. If the input is in another code page or encoding
+  and requires locale-specific processing (e.g. ignore/map case,
+  character classes), consider converting it through UTF-8 to ensure
+  the correct handling of locale-specific regular expressions.
+  This _sed_ program can also handle arbitrary byte sequences
+  if no part of the input requires treating it as a Rust String.
+* Back-references aren't supported when input is processed as bytes
+  (`LC_ALL=C`).
+* The command will report an error and fail if duplicate labels are found
+  in the script.
+  This matches the BSD behavior. The GNU version accepts duplicate labels.
+* The last line (`$`) address is interpreted as the last non-empty line of
+  the last file.  If files specified in subsequent arguments until the last
+  one are empty, then the last line condition will never be triggered.
+  This behavior is consistent with the
+  [original implementation](https://github.com/dspinellis/unix-history-repo/blob/Research-V7/usr/src/cmd/sed/sed1.c#L665).
+* Labels are parsed for alphanumeric characters. The BSD version parses them
+  until the end of the line, preventing ; to be used as a separator.

docs/src/index.md

@@ -0,0 +1,10 @@
+# uutils sed
+
+_sed_ is a Rust reimplementation of the
+[sed utility](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/sed.html)
+with some [GNU sed](https://www.gnu.org/software/sed/manual/sed.html),
+[FreeBSD sed](https://man.freebsd.org/cgi/man.cgi?sed(1)),
+and other extensions.
+
+It is part of the [uutils](https://uutils.github.io/) project. The source code
+is on [GitHub](https://github.com/uutils/sed).