What literal characters should be escaped in a regex?

Question

I just wrote a regex for use with the php function preg_match that contains the following part:

[\\w-.]

To match any word char

Answer 1

[\w.-]

• the . usually means any character but between [] has no special meaning
• - between [] indicates a range unless if it's escaped or either first or last character between []

Answer 2

While there are indeed some characters should be escaped in a regex, you're asking not about regex but about character class. Where dash symbol being special one.

instead of escaping it you could put it at the end of class, [\w.-]

Answer 3

The full stop loses its meta meaning in the character class.

The - has special meaning in the character class. If it isn't placed at the start or at the end of the square brackets, it must be escaped. Otherwise it denotes a character range (A-Z).

You triggered another special case however. [\w-.] works because \w does not denote a single character. As such PCRE can not possibly create a character range. \w is a possibly non-coherent class of symbols, so there is no end-character which could be used to create the range Z till .. Also the full stop . would preceed the first ascii character a that \w could match. There is no range constructable. Hencewhy - worked without escaping for you.

Answer 4

In many regex implementations, the following rules apply:

Meta characters inside a character class are:

• ^ (negation)
• - (range)
• ] (end of the class)
• \ (escape char)

So these should all be escaped. There are some corner cases though:

• - needs no escaping if placed at the very start, or end of the class ([abc-] or [-abc]). In quite a few regex implementations, it also needs no escaping when placed directly after a range ([a-c-abc]) or short-hand character class ([\w-abc]). This is what you observed
• ^ needs no escaping when it's not at the start of the class: [^a] means any char except a, and [a^] matches either a or ^, which equals: [\^a]
• ] needs no escaping if it's the only character in the class: []] matches the char ]

Answer 5

If you are using php and you need to escape special regex chars, just use preg_quote:

An example from php.net:

<?php
// In this example, preg_quote($word) is used to keep the
// asterisks from having special meaning to the regular
// expression.

$textbody = "This book is *very* difficult to find.";
$word = "*very*";
$textbody = preg_replace ("/" . preg_quote($word, '/') . "/",
                          "<i>" . $word . "</i>",
                          $textbody);
?>