From: Yuri Khan <yuri.v.khan@gmail.com>
To: Marcin Borkowski <mbork@wmi.amu.edu.pl>
Cc: "help-gnu-emacs@gnu.org" <help-gnu-emacs@gnu.org>
Subject: Re: How to grok a complicated regex?
Date: Sat, 14 Mar 2015 11:14:26 +0600 [thread overview]
Message-ID: <CAP_d_8UhjZABfO1yiBuz2sRMiUz+h3mk4O3g-VAKPLFs8AWapw@mail.gmail.com> (raw)
In-Reply-To: <87egosa3od.fsf@wmi.amu.edu.pl>
On Sat, Mar 14, 2015 at 5:16 AM, Marcin Borkowski <mbork@wmi.amu.edu.pl> wrote:
>>> "\\`\\(?:\\\\[([]\\|\\$+\\)?\\(.*?\\)\\(?:\\\\[])]\\|\\$+\\)?\\'"
>>>
> It's not really /difficult/.
> Intimidating, yes. Boring, possibly. Laborious (and mechanical), yes.
> But not /difficult/.
I tried it and it’s not very intimidating or boring or laborious or
difficult. Here’s my thought process:
First I unescape all backslashes, by global-replacing “\\” with “\”.
Then I insert spaces at key points to separate the syntactic
constructs. (Any literal spaces in the regexp need to be made
explicit, e.g. by replacing as <space>.)
\` \(?: \\ [([] \| \$+ \)? \(.*?\) \(?: \\ [])] \| \$+ \)? \'
Imagining the parentheses and alternatives as nested boxes might help, too:
┌─────────┬─────┐ ╔═══╗ ┌─────────┬─────┐
\` │ \\ [([] │ \$+ │? ║.*?║ │ \\ [])] │ \$+ │? \'
└─────────┴─────┘ ╚═══╝ └─────────┴─────┘
(Here the nesting level is just 1, so I didn’t actually need to draw
it, just match.)
Now I can read it:
1. start-of-string
2. optionally followed by either
* a backslash and either an opening parenthesis or bracket
* or one or more dollar signs
3. followed by any string, which is extracted as group 1
4. optionally followed by either
* a backslash and either a closing bracket or parenthesis
* or one or more dollar signs
5. followed by end-of-string
I can further grok it as matching a valid (La)TeX math formula: $…$,
$$…$$, \(…\), \[…\]; as well as some invalid markup such as $$$$…$$$,
$…\], \(…\], $$…, etc.
As for the bigger picture, I think, if a regular expression ends up
difficult to read, it needs decomposed into small, easily digestible
chunks, each with a descriptive name. Elisp has the let* form and the
rx macro for this purpose.
next prev parent reply other threads:[~2015-03-14 5:14 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.1979.1426282552.31049.help-gnu-emacs@gnu.org>
2015-03-13 22:46 ` How to grok a complicated regex? Emanuel Berg
2015-03-13 23:16 ` Marcin Borkowski
2015-03-14 0:12 ` Rasmus
2015-03-14 13:18 ` Stefan Monnier
[not found] ` <mailman.2003.1426339118.31049.help-gnu-emacs@gnu.org>
2015-03-15 4:31 ` Rusi
2015-03-22 2:29 ` Tom Tromey
2015-03-22 2:44 ` Rasmus
2015-03-14 5:14 ` Yuri Khan [this message]
2015-03-14 7:03 ` Drew Adams
[not found] ` <mailman.1984.1426288628.31049.help-gnu-emacs@gnu.org>
2015-03-14 3:58 ` Emanuel Berg
2015-03-14 4:44 ` Emanuel Berg
2015-03-14 4:58 ` Emanuel Berg
2015-03-14 8:43 ` Thien-Thi Nguyen
[not found] ` <mailman.1997.1426324089.31049.help-gnu-emacs@gnu.org>
2015-03-20 1:05 ` Emanuel Berg
2015-03-18 16:40 ` Alan Mackenzie
2015-03-19 8:15 ` Tassilo Horn
2015-04-25 4:23 ` Rusi
2015-04-27 13:26 ` Julien Cubizolles
2015-03-14 8:16 martin rudalics
-- strict thread matches above, loose matches on Subject: below --
2015-03-13 21:35 Marcin Borkowski
2015-03-13 21:45 ` Marcin Borkowski
2015-03-13 21:47 ` Alexis
2015-03-13 21:57 ` Marcin Borkowski
2015-03-23 12:18 ` Vaidheeswaran C
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