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Unit 5: Strings

Notes

ereg(“(123)+”, $var)
The Expression matches “123”, “123123”, “123123123”, and so on. Grouping characters allows
complex patterns to be expressed, as in the following example that matches an alphabetic-only URL:
// A simple, incomplete, HTTP URL regular expression
// that does not allow numbers
$pattern = ‘^(http://)?[a-zA-Z]+(\.[a-zA-z]+)+$’;
$found = ereg($pattern, “www.ora.com”); // true
Figure 5.1 shows the parts of this complex regular expression and how they are interpreted.
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 does not match any of
the subject string in the example but does not 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”.
Figure 5.1: Regular Expression with Groups

Groups can also define subpatterns when ereg( ) extracts values into an array.
Alternative Patterns

Alternatives in a pattern are specified with the | operator; for example, the pattern “cat|bat|rat”
matches “cat”, “bat”, or “rat”. The | operator has the lowest precedence of the regular expression
operators, treating the largest surrounding expressions as alternative patterns. To match “cat”,
“bat”, or “rat” another way, the following expression can be used:

$var = “bat”;
$found = ereg(“(c|b|r)at”, $var); // true
Another example shows alternative endings to a pattern:
// match some URL damains
$pattern = ‘(com$|net$|gov$|edu$)’;
$found = ereg($pattern, “http://www.ora.com”); // true

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