1 Overview
==========
`pcre2el' or `rxt' (RegeXp Translator or RegeXp Tools) is a utility
for working with regular expressions in Emacs, based on a
recursive-descent parser for regexp syntax. In addition to converting
(a subset of) PCRE syntax into its Emacs equivalent, it can do the
following:
- convert Emacs syntax to PCRE
- convert either syntax to `rx', an S-expression based regexp syntax
- untangle complex regexps by showing the parse tree in `rx' form and
highlighting the corresponding chunks of code
- show the complete list of strings (productions) matching a regexp,
provided the list is finite
- provide live font-locking of regexp syntax (so far only for Elisp
buffers -- other modes on the TODO list)
2 Usage
=======
Enable `rxt-mode' or its global equivalent `rxt-global-mode' to get
the default key-bindings. There are three sets of commands: commands
that take a PCRE regexp, commands which take an Emacs regexp, and
commands that try to do the right thing based on the current
mode. Currently, this means Emacs syntax in `emacs-lisp-mode' and
`lisp-interaction-mode', and PCRE syntax everywhere else.
The default key bindings all begin with `C-c /' and have a mnemonic
structure: `C-c / ', or just `C-c / ' for the
"do what I mean" commands. The complete list of key bindings is given
here and explained in more detail below:
- "Do-what-I-mean" commands:
`C-c / /': `rxt-explain'
`C-c / c': `rxt-convert-syntax'
`C-c / x': `rxt-convert-to-rx'
`C-c / '': `rxt-convert-to-strings'
- Commands that work on a PCRE regexp:
`C-c / p e': `rxt-pcre-to-elisp'
`C-c / %': `pcre-query-replace-regexp'
`C-c / p x': `rxt-pcre-to-rx'
`C-c / p '': `rxt-pcre-to-strings'
`C-c / p /': `rxt-explain-pcre'
- Commands that work on an Emacs regexp:
`C-c / e /': `rxt-explain-elisp'
`C-c / e p': `rxt-elisp-to-pcre'
`C-c / e x': `rxt-elisp-to-rx'
`C-c / e '': `rxt-elisp-to-strings'
`C-c / e t': `rxt-toggle-elisp-rx'
`C-c / t': `rxt-toggle-elisp-rx'
2.1 Interactive input and output
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When used interactively, the conversion commands can read a regexp
either from the current buffer or from the minibuffer. The output is
displayed in the minibuffer and copied to the kill-ring.
- When called with a prefix argument (`C-u'), they read a regular
expression from the minibuffer literally, without further processing
-- meaning there's no need to double the backslashes if it's an
Emacs regexp. This is the same way commands like
`query-replace-regexp' read input.
- When the region is active, they use they the region contents, again
literally (without any translation of string syntax).
- With neither a prefix arg nor an active region, the behavior depends
on whether the command expects an Emacs regexp or a PCRE one.
Commands that take an Emacs regexp behave like `C-x C-e': they
evaluate the sexp before point (which could be simply a string
literal) and use its value. This is designed for use in Elisp
buffers. As a special case, if point is *inside* a string, it's
first moved to the string end, so in practice they should work as
long as point is somewhere within the regexp literal.
Commands that take a PCRE regexp try to read a Perl-style delimited
regex literal *after* point in the current buffer, including its
flags. For example, putting point before the `m' in the following
example and doing `C-c / p e' (`rxt-pcre-to-elisp') displays
`\(?:bar\|foo\)', correctly stripping out the whitespace and
comment:
,----
| $x =~ m/ foo | (?# comment) bar /x
`----
The PCRE reader currently only works with `/ ... /' delimiters. It
will ignore any preceding `m', `s', or `qr' operator, as well as the
replacement part of an `s' construction.
Readers for other PCRE-using languages are on the TODO list.
The translation functions display their result in the minibuffer and
copy it to the kill ring. When translating something into Elisp
syntax, you might need to use the result either literally (e.g. for
interactive input to a command like `query-replace-regexp'), or as a
string to paste into Lisp code. To allow both uses,
`rxt-pcre-to-elisp' copies both versions successively to the
kill-ring. The literal regexp without string quoting is the top
element of the kill-ring, while the Lisp string is the
second-from-top. You can paste the literal regexp somewhere by doing
`C-y', or the Lisp string by `C-y M-y'.
2.2 Syntax conversion commands
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
`rxt-convert-syntax' (`C-c / c') converts between Emacs and PCRE
syntax, depending on the major mode in effect when called.
Alternatively, you can specify the conversion direction explicitly by
using either `rxt-pcre-to-elisp' (`C-c / p e') or `rxt-elisp-to-pcre'
(`C-c / e p').
Similarly, `rxt-convert-to-rx' (`C-c / x') converts either kind of
syntax to `rx' form, while `rxt-convert-pcre-to-rx' (`C-c / p x') and
`rxt-convert-elisp-to-rx' (`C-c / e x') convert to `rx' from a
specified source type.
In Elisp buffers, you can use `rxt-toggle-elisp-rx' (`C-c / t' or `C-c
/ e t') to switch the regexp at point back and forth between string
and `rx' syntax. Point should either be within an `rx' or
`rx-to-string' form or a string literal for this to work.
2.3 PCRE mode (experimental)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you want to use emulated PCRE regexp syntax in all Emacs commands,
try `pcre-mode', which uses Emacs's advice system to make all commands
that read regexps using the minibuffer use emulated PCRE syntax. It
should also work with Isearch.
This feature is still fairly experimental. It may fail to work or do
the wrong thing with certain commands. Please report bugs.
`pcre-query-replace-regexp' was originally defined to do query-replace
using emulated PCRE regexps, and is now made somewhat obsolete by
`pcre-mode'. It is bound to `C-c / %' by default, by analogy with
`M-%'. Put the following in your `.emacs' if you want to use
PCRE-style query replacement everywhere:
,----
| (global-set-key [(meta %)] 'pcre-query-replace-regexp)
`----
2.5 Explain regexps
~~~~~~~~~~~~~~~~~~~
When syntax-highlighting isn't enough to untangle some gnarly regexp
you find in the wild, try the 'explain' commands: `rxt-explain' (`C-c
/ /'), `rxt-explain-pcre' (`C-c / p') and `rxt-explain-elisp' (`C-c /
e'). These display the original regexp along with its pretty-printed
`rx' equivalent in a new buffer. Moving point around either in the
original regexp or the `rx' translation highlights corresponding
pieces of syntax, which can aid in seeing things like the scope of
quantifiers.
I call them "explain" commands because the `rx' form is close to a
plain syntax tree, and this plus the wordiness of the operators
usually helps to clarify what is going on. People who dislike Lisp
syntax might disagree with this assessment.
2.6 Generate all matching strings (productions)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Occasionally you come across a regexp which is designed to match a
finite set of strings, e.g. a set of keywords, and it would be useful
to recover the original set. (In Emacs you can generate such regexps
using `regexp-opt'). The commands `rxt-convert-to-strings' (`C-c /
′'), `rxt-pcre-to-strings' (`C-c / p ′') or `rxt-elisp-to-strings'
(`C-c / e ′') accomplish this by generating all the matching strings
("productions") of a regexp. (The productions are copied to the kill
ring as a Lisp list).
An example in Lisp code:
,----
| (regexp-opt '("cat" "caterpillar" "catatonic"))
| ;; => "\\(?:cat\\(?:atonic\\|erpillar\\)?\\)"
| (rxt-elisp-to-strings "\\(?:cat\\(?:atonic\\|erpillar\\)?\\)")
| ;; => '("cat" "caterpillar" "catatonic")
`----
For obvious reasons, these commands only work with regexps that don't
include any unbounded quantifiers like `+' or `*'. They also can't
enumerate all the characters that match a named character class like
`[[:alnum:]]'. In either case they will give a (hopefully meaningful)
error message. Due to the nature of permutations, it's still possible
for a finite regexp to generate a huge number of productions, which
will eat memory and slow down your Emacs. Be ready with `C-g' if
necessary.
2.7 RE-Builder support
~~~~~~~~~~~~~~~~~~~~~~
The Emacs RE-Builder is a useful visual tool which allows using
several different built-in syntaxes via `reb-change-syntax' (`C-c
TAB'). It supports Elisp read and literal syntax and `rx', but it can
only convert from the symbolic forms to Elisp, not the other way. This
package hacks the RE-Builder to also work with emulated PCRE syntax,
and to convert transparently between Elisp, PCRE and rx syntaxes. PCRE
mode reads a delimited Perl-like literal of the form `/ ... /', and it
should correctly support using the `x' and `s' flags.
2.8 Use from Lisp
~~~~~~~~~~~~~~~~~
Example of using the conversion functions:
,----
| (rxt-pcre-to-elisp "(abc|def)\\w+\\d+")
| ;; => "\\(\\(?:abc\\|def\\)\\)[_[:alnum:]]+[[:digit:]]+"
`----
All the conversion functions take a single string argument, the regexp
to translate:
- `rxt-pcre-to-elisp'
- `rxt-pcre-to-rx'
- `rxt-pcre-to-strings'
- `rxt-elisp-to-pcre'
- `rxt-elisp-to-rx'
- `rxt-elisp-to-strings'
3 Bugs and Limitations
======================
3.1 Limitations on PCRE syntax
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PCRE has a complicated syntax and semantics, only some of which can be
translated into Elisp. The following subset of PCRE should be
correctly parsed and converted:
- parenthesis grouping `( .. )', including shy matches `(?: ... )'
- backreferences (various syntaxes), but only up to 9 per expression
- alternation `|'
- greedy and non-greedy quantifiers `*', `*?', `+', `+?', `?' and `??'
(all of which are the same in Elisp as in PCRE)
- numerical quantifiers `{M,N}'
- beginning/end of string `\A', `\Z'
- string quoting `\Q .. \E'
- word boundaries `\b', `\B' (these are the same in Elisp)
- single character escapes `\a', `\c', `\e', `\f', `\n', `\r', `\t',
`\x', and `\octal digits' (but see below about non-ASCII characters)
- character classes `[...]' including Posix escapes
- character classes `\d', `\D', `\h', `\H', `\s', `\S', `\v', `\V'
both within character class brackets and outside
- word and non-word characters `\w' and `\W' (Emacs has the same
syntax, but its meaning is different)
- `s' (single line) and `x' (extended syntax) flags, in regexp
literals, or set within the expression via `(?xs-xs)' or `(?xs-xs:
.... )' syntax
- comments `(?# ... )'
Most of the more esoteric PCRE features can't really be supported by
simple translation to Elisp regexps. These include the different
lookaround assertions, conditionals, and the "backtracking control
verbs" `(* ...)' . OTOH, there are a few other syntaxes which are
currently unsupported and possibly could be:
- `\L', `\U', `\l', `\u' case modifiers
- `\g{...}' backreferences
3.2 Other limitations
~~~~~~~~~~~~~~~~~~~~~
- The order of alternatives and characters in char classes sometimes
gets shifted around, which is annoying.
- Although the string parser tries to interpret PCRE's octal and
hexadecimal escapes correctly, there are problems with matching
8-bit characters that I don't use enough to properly understand,
e.g.:
,----
| (string-match-p (rxt-pcre-to-elisp "\\377") "\377") => nil
`----
A fix for this would be welcome.
- Most of PCRE's rules for how `^', `\A', `$' and `\Z' interact with
newlines are not implemented, since they seem less relevant to
Emacs's buffer-oriented rather than line-oriented model. However,
the different meanings of the `.' metacharacter *are* implemented
(it matches newlines with the `/s' flag, but not otherwise).
- Not currently namespace clean (both `rxt-' and a couple of `pcre-'
functions).
3.3 TODO:
~~~~~~~~~
- Python-specific extensions to PCRE?
- Language-specific stuff to enable regexp font-locking and explaining
in different modes. Each language would need two functions, which
could be kept in an alist:
1. A function to read PCRE regexps, taking the string syntax into
account. E.g., Python has single-quoted, double-quoted and raw
strings, each with different quoting rules. PHP has the kind of
belt-and-suspenders solution you would expect: regexps are in
strings, /and/ you have to include the `/ ... /' delimiters!
Duh.
2. A function to copy faces back from the parsed string to the
original buffer text. This has to recognize any escape sequences
so they can be treated as a single character.
4 Internal details
==================
`rxt' defines an internal syntax tree representation of regular
expressions, parsers for Elisp and PCRE syntax, and 'unparsers'
to convert the internal representation to PCRE or `rx' syntax.
Converting from the internal representation to Emacs syntax is
done by converting to `rx' form and passing it to `rx-to-string'.
See `rxt-parse-re', `rxt-adt->pcre', and `rxt-adt->rx' for
details.
This code is partially based on Olin Shivers' reference SRE
implementation in scsh, although it is simplified in some respects and
extended in others. See `scsh/re.scm', `scsh/spencer.scm' and
`scsh/posixstr.scm' in the `scsh' source tree for details. In
particular, `pcre2el' steals the idea of an abstract data type for
regular expressions and the general structure of the string regexp
parser and unparser. The data types for character sets are extended in
order to support symbolic translation between character set
expressions without assuming a small (Latin1) character set. The
string parser is also extended to parse a bigger variety of
constructions, including POSIX character classes and various Emacs and
Perl regexp assertions. Otherwise, only the bare minimum of scsh's
abstract data type is implemented.
5 Soapbox
=========
Emacs regexps have their annoyances, but it is worth getting used to
them. The Emacs assertions for word boundaries, symbol boundaries, and
syntax classes depending on the syntax of the mode in effect are
especially useful. (PCRE has `\b' for word-boundary, but AFAIK it
doesn't have separate assertions for beginning-of-word and
end-of-word). Other things that might be done with huge regexps in
other languages can be expressed more understandably in Elisp using
combinations of `save-excursion' with the various searches (regexp,
literal, skip-syntax-forward, sexp-movement functions, etc.).
There's not much point in using `rxt-pcre-to-elisp' to use PCRE
notation in a Lisp program you're going to maintain, since you still
have to double all the backslashes. Better to just use the converted
result (or better yet, the `rx' form).
6 History and acknowledgments
=============================
This was originally created out of an answer to a stackoverflow
question:
[http://stackoverflow.com/questions/9118183/elisp-mechanism-for-converting-pcre-regexps-to-emacs-regexps]
Thanks to:
- Wes Hardaker (hardaker) for the initial inspiration and subsequent
hacking
- priyadarshan for requesting RX support
- Daniel Colascione (dcolascione) for a patch to support Emacs's
explicitly-numbered match groups
- Aaron Meurer (asmeurer) for requesting Isearch support
- Philippe Vaucher (silex) for a patch to support `ibuffer-do-replace-regexp'
in PCRE mode