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

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CHAPTER 9 ■ PERFORMING FORM VALIDATION WITH REGULAR EXPRESSIONS
The only difference between str_replace() and preg_replace() on a basic level is that the element
passed to preg_replace() for the pattern must use delimiters, which let the function know which part of
the regex is the pattern and which part consists of modifiers, or flags that affect how the pattern matches.
You’ll learn more about modifiers a little later in this section.
The delimiters for regex patterns in preg_replace() can be any non-alphanumeric, non-backslash,
and non-whitespace characters placed at the beginning and end of the pattern. Most commonly,
forward slashes (/) or hash signs (#) are used. For instance, if you want to search for the letters cat in a
string, the pattern would be /cat/ (or #cat#, %cat%, @cat@, and so on).
Choosing Regexes vs. Regular String Replacement
To explore the differences between str_replace() and preg_replace(), try using both functions to wrap
any occurrence of the word regular with tags. Make the following modifications to regex.php:
Regular Expression Demo
em {
background-color: #FF0;
border-top: 1px solid #000;
border-bottom: 1px solid #000;
}