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Re: Unquoted special characters in regexps

From: martin rudalics
Subject: Re: Unquoted special characters in regexps
Date: Fri, 03 Mar 2006 16:51:52 +0100
User-agent: Mozilla Thunderbird 1.0 (Windows/20041206)

>         "\\(\[[0-9]+\] \\)*\\([a-zA-Z0-9.$_]+\\)\\.[a-zA-Z0-9$_<>(),]+ \
>     \\(([a-zA-Z0-9.$_]+:\\|line=\\)\\([0-9.,]+\\)"
>     Experience tells me that this should be probably written as
>         "\\(\\[[0-9]+\\] \\)*\\([a-zA-Z0-9.$_]+\\)\\.[a-zA-Z0-9$_<>(),]+ \
>     \\(([a-zA-Z0-9.$_]+:\\|line=\\)\\([0-9.,]+\\)"
> \[ and \] in Lisp strings are equivalent to just [ and just ].  So I
> think the current value is incorrect, and the [ needs to have \\ before it.
> Meanwhile, the question we're discussing here is whether to write \\
> before the ].  That is harmless, and the question is whether it makes
> things clearer or more confusing.  The problem is that usually it
> makes things clearer, but occasionally people could get confused when
> \\ is last in a character alternative.

The question whether writing '\\' before the `]' is relevant for the
example cited above.  Usually, when I see a `\\]' outside a character
alternative I expect it to match a right bracket in some text.  And,
usually, in that text a left bracket will precede the right bracket.
Hence, if in the text above the author had used `\\]' instead of `\]' it
would have been easy to conclude - from the absence of a preceding `\\['
- that something went wrong.  Vice versa, when seeing a `\\[' I usually
expect it to have a corresponding `\\]' somehwere on the right.

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