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