正則表達式在PHP中被用來處理復雜的文字串。支持正則表達式的函數有：
ereg()
ereg replace()
eregi replace()
split()
這些函數都將正則表達式作為他們的第一個參數。PHP使用POSIX擴展規則表達式（使用POSIX 1003.2）。要找到所有的關于POSIX擴展規則表達式的描述，請查看包括在PHP發行版本之內的regex man頁面。
Example 2-4. Regular expression examples
ereg("abc",$string);
/* Returns true if "abc"
is found anywhere in $string. */
ereg("^abc",$string);
/* Returns true if "abc"
is found at the beginning of $string. */
ereg("abc$",$string);
/* Returns true if "abc"
is found at the end of $string. */
eregi("(ozilla.[23]|MSIE.3)",$HTTP_USER_AGENT);
/* Returns true if client browser
is Netscape 2, 3 or MSIE 3. */
ereg("([[:alnum:]]+) ([[:alnum:]]+) ([[:alnum:]]+ )",
$string,$regs);
/* Places three space separated words
into $regs[1], $regs[2] and $regs[3]. */
ereg_replace("^","<BR>",$string);
/* Put a <BR> tag at the beginning of $string. */
ereg_replace("$","<BR>",$string);
/* Put a <BR> tag at the end of $string. */
ereg_replace("\n","",$string);
/* Get rid of any carriage return
characters in $string. */
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