Programming PHP

Inline Options
In addition to specifying patternwide options after the closing pattern delimiter, you
can specify options within a pattern to have them apply only to part of the pattern.
The syntax for this is:
(?flags:subpattern)
For example, only the word “PHP” is case-insensitive in this example:
preg_match('/I like (?i:PHP)/', 'I like pHp'); // returns true
The i, m, s, U, x, and X options can be applied internally in this fashion. You can use
multiple options at once:
preg_match('/eat (?ix:fo o d)/', 'eat FoOD'); // returns true
Prefix an option with a hyphen (-) to turn it off:
preg_match('/(?-i:I like) PHP/i', 'I like pHp'); // returns true
An alternative form enables or disables the flags until the end of the enclosing subpattern
or pattern:
preg_match('/I like (?i)PHP/', 'I like pHp'); // returns true
preg_match('/I (like (?i)PHP) a lot/', 'I like pHp a lot', $match);
// $match[1] is 'like pHp'
Inline flags do not enable capturing. You need an additional set of capturing parentheses
do that.
Lookahead and Lookbehind
It’s sometimes useful in patterns to be able to say “match here if this is next.” This is
particularly common when you are splitting a string. The regular expression
describes the separator, which is not returned. You can use lookahead to make sure
(without matching it, thus preventing it from being returned) that there’s more data
after the separator. Similarly, lookbehind checks the preceding text.
Lookahead and lookbehind come in two forms: positive and negative. A positive lookahead
or lookbehind says “the next/preceding text must be like this.” A negative lookahead
or lookbehind says “the next/preceding text must not be like this.” Table 4-14
shows the four constructs you can use in Perl-compatible patterns. None of the constructs
captures text.
Table 4-14. Lookahead and lookbehind assertions
Construct Meaning
(?=subpattern) Positive lookahead
(?!subpattern) Negative lookahead
(?