Nesting [ ] with PHP RegExp

Question

I'm trying to ensure that a string in PHP has only letters, hyphens or apostraphes. To accomplish this I wanted to make a range of valid characters using [ ]. So my idea was to do this:

[[A-Za-z]-'] // Weird syntax highlighting here

Will this work? Is it possible to nest brackets like that? This is meant to match a single character that is either a letter, a hyphen, or an apostraphe. I may be approaching the problem naively and that's OK, I just wanted to know if putting brackets within brackets like this is legal in PHP. Thanks!

I think you can also benefit from this nice overview of available regex design tools: stackoverflow.com/questions/89718/… — mario
– mario, Commented Apr 30, 2011 at 22:35

David Z · Accepted Answer · 2011-05-01 00:35:28Z

3

I'm assuming you're using this in one of the regular expression matching functions (like preg_match("[[A-Za-z]-']*", ...), and in that case, it's a question not of PHP syntax, but regular expression syntax. And the answer is no, you can't nest brackets like that. If you want a regular expression that matches only a letter, hyphen, or apostrophe, use [A-Za-z'-]. (The hyphen goes last so that the regex engine knows that it's not representing a range of characters like A-Z. Alternatively you can escape the hyphen with a backslash, then you can put it anywhere: [A-Za-z\-'].)

edited May 1, 2011 at 0:35

answered Apr 30, 2011 at 22:32

David Z

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6 Comments

Nick Van Hoogenstyn Over a year ago

Ah, perfect. I was making it unnecessarily complicated! Thanks.

Rudie Over a year ago

Just escape the hyphen. Always escape the hyphens, so you never forget to. Saves you an hour in a lifetime =)

David Z Over a year ago

@Rudie: good advice, I suppose. I've just gotten used to putting the hyphen at the end whenever I need it in a character class.

Rudie Over a year ago

That works just as well. I was learned to always escape it. Just to be safe. In the end it doesn't matter I guess, as long as it works the way you want.

tchrist Over a year ago

That’s not a letter. It’s A-Z.

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Lightness Races in Orbit · Accepted Answer · 2011-04-30 22:33:11Z

1

I don't understand.

What's wrong with

[A-Za-z'-]

?

answered Apr 30, 2011 at 22:33

Lightness Races in Orbit

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tchrist · Accepted Answer · 2011-05-01 01:50:12Z

0

[\pL\p{Pd}'ʹ’]⁠ ⁠

That matches:

any Letter character⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠ ⁠
any Dash Punctuation character
U+0027 APOSTROPHE (which is not the preferred form)
U+02B9 MODIFIER LETTER PRIME
U+2019 RIGHT SINGLE QUOTATION MARK

edited May 1, 2011 at 1:50

answered May 1, 2011 at 1:18

tchrist

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Comments

ridgerunner · Accepted Answer · 2011-05-01 02:34:25Z

To ensure that a string contains only the desired characters you can do it two ways:

You know its good if all chars in the string are valid.
You know its bad if any one char in the string is invalid.

Here is a PHP snippet that demonstrates both methods:

// Method 1: Good if all chars in the string are valid.
$re_all_valid = '/^[A-Za-z\-\']*$/';
if (preg_match($re_all_valid, $text)) {
    echo("GOOD: String contains all valid characters.\n");
} else {
    echo("BAD: String does NOT contain all valid characters.\n");
}

// Method 2: Bad if any one char in the string is invalid.
$re_one_invalid = '/[^A-Za-z\-\']/';
if (preg_match($re_one_invalid, $text)) {
    echo("BAD: String contains one invalid character.\n");
} else {
    echo("GOOD: String does NOT contain one invalid character.\n");
}

Notes: Method 1 requires anchors at both ends of the string and a quantifier applied to the positive character class. Method 2 uses a negated character class and only needs to match one character in the string. Method 2 is likely more efficient.

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