[wikka-community] Coding guideline about backslash.

Mahefa Randimbisoa dotmg
Tue Oct 9 05:25:16 GMT 2007

No Marjolein, I'm sorry.

 echo '\\n';
will output \n, a single backslash and the letter n.

echo '\\\\\\d$' will give \\\d$, that's really why I chose that example.
It will lead you to confusion if you think backslashes need not be escaped inside single-quote-delimited strings.

Inside a string delimited by a single quote, there are 2 characters that can (and need) be escaped with a backslash: a single quote or a backslash.
The reason is simple: how could you use single quote to delimit a string that ends with a singlequote, and how would you do for a string
that ends with a backslash? 
For the first one, you write '...\'' (the single quote escaped by backslash represents a single single-quote, and the final single-quote delimits the string) 
and for the second one: '...\\' 
(The backslash need to be escaped, otherwise, the parser will consider \' as an escaped single-quote, and expects another single-quote to delimit the string.

Convinced with this explanation, and as you have mentionned, things get more complicated with regular expressions.
If you want a regex to test a string starting with the 2 characters \ and n, you really need to represent it with 4 backslashes like 
'/^\\\\n/', __even if you use singlequote__
. Other representations will fail.

Ex: '/^\\n' - php will pass the string ^\n to the regexp, and regex will consider it as a pattern that match something starting with a LF.

preg_quote() is a handy function, but you some string representations like \s, \d should not be passed to it (when you intend their use as regexp special character, in this case a character string or a digit).



> Date: Mon, 8 Oct 2007 17:31:41 +0200
> To: community at wikkawiki.org
> From: javawoman at wikkawiki.org
> Subject: Re: [wikka-community] Coding guideline about backslash.
> At 14:09 2007-10-08, Mahefa wrote:
> >Which coding guideline about backslash?
> >
> >My coding style for writing a backslash inside a string, whether
> >double quote or single quote is used as delimiter; is to expressly
> >escape it with another backslash.
> >
> >These 2 strings are the same to write a string composed of 2
> >characters: a backslash and the letter n.
> >'\\n' and '\n'
> Actually they are NOT the same: within single quotes, every character is 
> just a literal character, so there is nothing to "escape". So '\\n' reads 
> "two backslashes and the letter n" and '\n' reads "a backslash and the 
> letter n".
> It's not a matter of a rule for backslashes, but a rule for using single or 
> double quotes:
> - use single quotes for LITERALS (every character stands for itself)
> - use double quotes only for strings that need to be INTERPOLATED
> It's only in "interpolated" strings  that you may need an escape character 
> to make a "special" character stand for itself instead of something to be 
> interpolated.
> >I prefer the first one, and the reasons are:
> >
> >1) When in the future, someone changes my singlequote in doublequote,
> >errors due to this change are minimized.
> When someone changes the single quotes to double quotes they must have a 
> reason for that - and that brings with it the responsibility to consider 
> whether any character may need to be escaped.
> >2) clarity: I don't need to think if the character that follow the
> >backslash has a special meaning when eventually combined with it. I
> >just have to count the number of backslashes and divide them by 2.
> Yes, you DO need to think (not to divide by two but to escape special 
> characters), because you need to make a reasoned decision to use double 
> quotes in the first place, instead of the generally preferred single 
> quotes. If you just need one or two interpolated characters, it's better to 
> concatenate them with the rest of the string still in single quotes.
> So if you start with:
>          echo 'This is a very looong string to be written to the output.';
> and want to add a newline to that, the solution is NOT
>          echo "This is a very looong string to be written to the output.\n";
> but instead:
>          echo 'This is a very looong string to be written to the output.'."\n";
> >Consider you want to write a constant \\\d$, using a single
> >quote as delimiter.
> >
> >If you use '\\\d$', the string will be : \\d$
> No it won't - it will be \\\d$ because every character stands 
> for itself in single quotes: it's a LITERAL.
> >You can easily get in trouble if you used to consider that escaping
> >backslash is not needed within single-quote-delimited strings.
> That's because escaping a backslash ISN'T needed for literals. The idea is 
> even meaningless, because there is nothing to escape. So if you "used to 
> consider" that, please go right on doing so, because it's true.
> >To write the string correctly, you must do one of the 4 proposals below.
> >'\\\\d$' or '\\\\\d$' or '\\\\\d$' or
> >'\\\\\\d$'
> >
> >For me, having in mind that I always escape my backslashes, the 4th
> >proposal is what I can read and understand more easily.
> Actually, I find that hard to read - if it would be in double quotes - 
> unless you really intend it to be "four backslashes, an IP address, two 
> backslashes, the letter d and a dollar sign".
> That said, there is only one special case to consider and that is using 
> such strings as (building blocks for) regular expressions - because regular 
> expressions have their own layer of interpolating and escaping: the PHP 
> engine is handing off the string to the PCRE engine which *itself* 
> interprets the regex string it is handed (essentially only a character 
> class has "literal" values (except for the dash which will be a literal if 
> put at the end), everything else is interpolated).
> In that case the simple solution is to still write the string as you mean 
> to - as a literal whenever possible -, keeping it readable for humans, and 
> use preg_quote() to let the PCRE library do its own escaping.
> To summarize:
> - use single quotes for literals, as much as possible
> - use double quotes ONLY when you need strings to be interpolated (by PHP), 
> and whenever feasible concatenate only those bits with single-quoted literals
> That much is pure PHP.
> For regular expressions you need to consider how those strings are going to 
> be used:
> - when using regular expressions, use PCRE (preg_*) in PHP: that keeps the 
> syntax consistent, expressions usable as building blocks, and our RE 
> library (in the making) better maintainable
> - when *creating* regular expressions, use the above rules for strings
> - when the *resulting* string (after PHP has done any interpolation 
> already!) /might/ contain any character that is a "special character" in 
> PCRE, use preg_quote() to escape it before passing it to the PCRE engine 
> with one of the (other) preg_* functions. See http://php.net/preg-quote.php .
> I don't think the last bits are on our coding guidelines page yet - I'll 
> add that soon.
> >--
> >Mahefa Randimbisoa (aka DotMG)
> >
> >_______________________________________________
> >WikkaWiki Community mailing list
> >community at wikkawiki.org
> >http://mail.wikkawiki.org/mailman/listinfo/community_wikkawiki.org
> --
> JavaWoman
> Web Standards Compliance Officer, Wikka Development Crew
> http://wikkawiki.org/JavaWoman
> Skype: callto://goneagain
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> http://mail.wikkawiki.org/mailman/listinfo/community_wikkawiki.org

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