Leaning toothpick syndrome
From Wikipedia, the free encyclopedia
In computer programming, leaning toothpick syndrome (LTS) is the situation in which a quoted expression becomes unreadable because it contains a large number of Escape characters, usually backslashes ("\") to avoid delimiter collision.
The official Perl documentation[1] introduced the term into wide usage; there, the phrase is used to describe regular expressions which match Unix-style path names, in which elements are separated by forward slashes.
LTS appears in many programming languages and in many situations, including:
- Patterns which match Uniform Resource Identifiers (URIs)
- Output of HTML
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[edit] Pattern example
Consider the following Perl regular expression intended to match URIs which identify files under the pub directory of an FTP site:
m/ftp:\/\/[^\/]*\/pub\//
Perl solves this problem by allowing many other characters to be delimiters for a regular expression. For example, the following three examples are equivalent to the expression given above:
m{ftp://[^/]*/pub/} m#ftp://[^/]*/pub/# m!ftp://[^/]*/pub/!
[edit] HTML example
A program is supposed to output an HTML-Link, the URL and the text for the link are found in variables $url and $text respectively:
print "<a href=\"$url\">$text</a>";
Using single quotes to delimit the string is not feasible: the variables canot be used inside single quotes:
print '<a href="$url">$text</a>'
Using the funktion printf is a viable solution in many languages (Perl, C, PHP):
printf('<a href="%s">%s</a>', $url, $text);
The qq-Operator in Perl allows for any delimiter:
print qq{'<a href="$url">$text</a>'}; print qq|'<a href="$url">$text</a>'|; print qq/'<a href="$url">$text</a>'/;
The here-doc is especially well suited for multiline strings and not suited for proper Indentation. This example shows the Perl-Syntax
print <<HERE_IT_ENDS; <a href="$url">$text</a> HERE_IT_ENDS
The same example in PHP-Syntax:
echo <<<HERE_IT_ENDS <a href="$url">$text</a> HERE_IT_ENDS;
