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正则表达式在PHP中被用来处理复杂的文字串。支持正则表达式的函数有：
ereg()ereg replace()eregi replace()split()
这些函数都将正则表达式作为他们的第一个参数。PHP使用POSIX扩展规则表达式（使用POSIX 1003.2）。要找到所有的关于POSIX扩展规则表达式的描述，请查看包括在PHP发行版本之内的regex man页面。
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 browseris Netscape 2, 3 or MSIE 3. */ereg("([[:alnum:]]+) ([[:alnum:]]+) ([[:alnum:]]+ )",$string,$regs);/* Places three space separated wordsinto $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 $string. */ereg_replace("
","",$string);/* Get rid of any carriage returncharacters in $string. */
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