Pro PHP and jQuery by Jason Lengstorf.pdf - Computer Science ...

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CHAPTER 9 ■ PERFORMING FORM VALIDATION WITH REGULAR EXPRESSIONS
Getting Fancy with Backreferences
The power of regexes starts to appear when you apply one of their most useful features: grouping and
backreferences. A group is any part of a pattern that is enclosed in parentheses. A group can be used in
the replacement string (or later in the pattern) with a backreference, a numbered reference to a named
group.
This all sounds confusing, but in practice it’s quite simple. Each set of parentheses from left to right
in a regex is stored with a numeric backreference, which can be accessed using a backslash and the
number of the backreference (\1) or by using a dollar sign and the number of the backreference ($1).
The benefit of this is that it gives regexes the ability to use the matched value in the replacement,
instead of a predetermined value as in str_replace() and its ilk.
To keep the replacement contents in your previous example in the proper case, you need to use two
occurrences of str_replace(); however, you can achieve the same effect by using a backreference in
preg_replace()with just one function call.
Make the following modifications to regex.php to see the power of backreferences in regexes:
Regular Expression Demo
em {
background-color: #FF0;
border-top: 1px solid #000;
border-bottom: 1px solid #000;
}