2.7.1.1. Characters and wildcards

The use of the backslash in a regular expression can cause confusion. To include a backslash in a double-quoted string, you need to escape the meaning of the backslash with a backslash. The following example shows how the regular expression pattern "\.com" is represented:

// Sets $found to true
$found = ereg("\\.com", "www.ora.com"); 

It's better to avoid the confusion and use single quotes when passing a string as a regular expression:

$found = ereg('\.com', "www.ora.com"); 

2.7.1.2. Character lists

ereg("p[aeiou]p", $var) 

can be used. This returns true for any string that contains "pap", "pep", "pip", "pop", or "pup". A range of characters can also be specified; for example, "[0-9]" specifies the numbers 0 through 9:

// Matches "A1", "A2", "A3", "B1", ...
$found = ereg("[ABC][123]", "A1 Quality");  // true

// Matches "00" to "39"
$found = ereg("[0-3][0-9]", "27");  //true

A list can specify characters that aren't matches using the not operator ^ as the first character in the brackets. The pattern "[^123]" matches any character other than 1, 2, or 3. The following examples show more regular expressions that make use of the not operator in lists:

// true for "pap", "pbp", "pcp", etc. but not "php"
$found = ereg("p[^h]p", $val);

// true if $var does not contain 
// alphanumeric characters
$found = ereg("[^0-9a-zA-Z]", $val);

The ^ character can be treated as normal by placing it in a position other than the start of the characters enclosed in the brackets. For example, "[0-9^]" matches the characters 0 to 9 and the ^ character. The - character can be matched by placing it at the start or the end of the list; for example, "[-123]" matches characters -, 1, 2, or 3.

2.7.1.3. Anchors

 ereg("^php", $var) 

matches strings that start with "php" but not others. The following code shows the operation of both:

$var = "to be or not to be";

$match = ereg("^to", $var); // true
$match = ereg('be$', $var); // true
$match = ereg("^or", $var); // false

Both anchors can be used in one regular expression to match a whole string. The following example illustrates this:

// Must match "Yes" exactly
$match = ereg('^Yes$', "Yes");     // true
$match = ereg('^Yes$', "Yes sir"); // false

2.7.1.4. Optional and repeating characters

The ? operator allows zero or one occurrence of a character, so the expression:

 ereg("pe?p", $var) 

matches either "pep" or "pp", but not the string "peep". The * operator allows zero or many occurrences of the "o" in the expression:

 ereg("po*p", $var) 

and matches "pp", "pop", "poop", "pooop", and so on. Finally, the + operator allows one to many occurrences of "b" in the expression:

 ereg("ab+a", $var)

so while strings such as "aba", "abba", and "abbba" match, "aa" doesn't.

The operators ?, *, and + can also be used with a wildcard or a list of characters. The following examples show how:

$var = "www.rmit.edu.au";

// True for strings that start with "www" 
// and end with "au"
$matches = ereg('^www.*au$', $var); // true


$hexString = "x01ff";

// True for strings that start with 'x' 
// followed by at least one hexadecimal digit
$matches = ereg('x[0-9a-fA-F]+$', $hexString); // true

The first example matches any string that starts with "www" and ends with "au"; the pattern ".*" matches a sequence of any characters, including a blank string. The second example matches any sequence that starts with the character "x" followed by one or more characters from the list [0-9a-fA-F].

A fixed number of occurrences can be specified in braces. for example, the pattern "[0-7]{3}" matches three-character numbers that contain the digits 0 through 7:

$valid = ereg("[0-7]{3}", "075"); // true
$valid = ereg("[0-7]{3}", "75");  // false

The braces syntax also allows the minimum and maximum occurrences of a pattern to be specified as demonstrated in the following examples:

$val = "58273";

// true if $val contains numerals from start to end
// and is between 4 and 6 characters in length
$valid = ereg('^[0-9]{4,6}$', $val); // true

$val = "5827003";
$valid = ereg('^[0-9]{4,6}$', $val); // false

// Without the anchors at the start and end, the 
// matching pattern "582768" is found
$val = "582768986456245003";

$valid = ereg("[0-9]{4,6}", $val);   // true

2.7.1.5. Groups

 ereg("(123)+", $var) 

matches "123", "123123", "123123123", etc. Grouping characters allows complex patterns to be expressed, as in the following example that matches a URL:

// A simple, incomplete, HTTP URL regular expression that doesn't allow numbers
$pattern = '^(http://)?[a-zA-Z]+(\.[a-zA-z]+)+$';

$found = ereg($pattern, "www.ora.com"); // true

The regular expression assigned to $pattern includes both the start and end anchors, ^ and $, so the whole subject string, "www.ora.com" must match the pattern. The start of the pattern is the optional group of characters "http://", as specified by "(http://)?". This doesn't match any of the subject string in the example but doesn't rule out a match, because the "http://" pattern is optional. Next the "[a-zA-Z]+" pattern specifies one or more alpha characters, and this matches "www" from the subject string. The next pattern is the group "(\.[a-zA-z]+)". This pattern must start with a period—the wildcard meaning of . is escaped with the backslash—followed by one or more alphabetic characters. The pattern in this group is followed by the + operator, so the pattern must occur at least once in the subject and can repeat many times. In the example, the first occurrence is ".ora" and the second occurrence is ".com".

Groups can also define subpatterns when ereg( ) extracts values into an array. We discuss the use of ereg( ) to extract values later in this section.

2.7.1.6. Alternative patterns

$var = "bat";
$found = ereg("(c|b|r)at", $var);  // true

Another example shows alternative beginnings to a pattern:

// match some URLs
$pattern = '(^ftp|^http|^gopher)://';

$found = ereg($pattern, "http://www.ora.com"); // true

2.7.1.7. Escaping special characters

// need to escape ( and )
$phone = "(03) 9429 5555";
$found = ereg("^\([0-9]{2,3}\)", $phone); // true

// No need to escape (*.+?)| within parentheses
$special = "Special Characters are (, ), *, +, ?, |";
$found = ereg("[(*.+?)|]", $special); // true

// The back-slash always needs to be quoted to match
$backSlash = 'The backslash \ character';
$found = ereg('^[a-zA-Z \\]*$', $backSlash); //true

// Don't need to escape the dot within parentheses
$domain = "www.ora.com";
$found = ereg("[.]com", $domain); //true

Another complication arises due to the fact that a regular expression is passed as a string to the regular expression functions. Strings in PHP can also use the backslash character to escape quotes and to encode tabs, newlines, etc. Consider the following example, which matches a backslash character:

// single-quoted string containing a backslash
$backSlash = '\ backslash';

// Evaluates to true 
$found = ereg("^\\\\ backslash\$", $backSlash);

The regular expression looks quite odd: to match a backslash, the regular expression function needs to escape the meaning of backslash, but because we are using a double-quoted string, each of the two backslashes needs to be escaped. The last complication is that PHP interprets the $ character as the beginning of a variable name, so we need to escape that. Using a single-quoted string can help make regular expressions easier to read and write.

2.7.1.8. Metacharacters

$source = "fast\tfood";

$result = ereg('\s', $source); // true

2.7.2.1. Finding and extracting values

boolean ereg(string pattern, string subject [, array var])
boolean eregi(string pattern, string subject [, array var])

Both functions return true if the regular expression pattern is found in the subject string. An optional array variable var can be passed as the third argument; it is populated with the portions of subject that are matched by up to nine grouped subexpressions in pattern. Both functions return false if the pattern isn't found in the subject.

To extract values from a string into an array, patterns can be arranged in groups contained by parentheses in the regular expression. The following example shows how the year, month, and day components of a date can be extracted into an array:

$parts = array( );
$value = "2001-09-07";
$pattern = '^([0-9]{4})-([0-9]{2})-([0-9]{2})$';

ereg($pattern, $value, $parts);

// Array ([0]=> 2001-09-07 [1]=>2001 [2]=>09 [3]=>07 
print_r($parts); 

The expression:

'^([0-9]{4})-([0-9]{2})-([0-9]{2})$'

matches dates in the format YYYY-MM-DD. After calling ereg( ), $parts[0] is assigned the portion of the string that matches the whole regular expression—in this case, the whole string 2001-09-07. The portion of the date that matches each group in the expression is assigned to the following array elements: $parts[1] contains the year matched by ([0-9]{4}), $parts[2] contains the month matched by ([0-9]{2}), and $parts[3] contains the day matched by "([0-9]{2})".

2.7.2.2. Replacing substrings

string ereg_replace(string pattern, string replacement, string source)
string eregi_replace(string pattern, string replacement, string source)

2.7.2.3. Splitting a string into an array

array split(string pattern, string source [, integer limit])
array spliti(string pattern, string source [, integer limit])

$sentence = "I wonder why he does\nBuzz, buzz, buzz!";
$words = split("[^a-zA-Z]+", $sentence);