规则：

• 允许字符和数字（a-z，A-Z，0-9）
• 允许下划线

正则表达式：

/^[a-zA-Z0-9_]{3,16}$/

代码示例：

function validate_username( $username ) {
if(preg_match(‘/^[a-zA-Z0-9_]{3,16}$/’, $_GET['username'])) {
return true;
}
return false;
}

2、匹配XHTML或XML标签

正则表达式：

{]*>(.*?)}

代码示例：

function get_tag( $tag, $xml ) {
$tag = preg_quote($tag);
preg_match_all(‘{<'.$tag.'[^>]*>(.*?).’}',
$xml,
$matches,
PREG_PATTERN_ORDER);

return $matches[1];
}

3、匹配确定属性值的XHTML或XML标签（例如：class或tag）

正则表达式：

{]*attribute\\s*=\\s*(["'])value\\\\1[^>]*>(.*?)}

代码示例：

function get_tag( $attr, $value, $xml, $tag=null ) {
if( is_null($tag) )
$tag = ‘\\w+’;
else
$tag = preg_quote($tag);

$attr = preg_quote($attr);
$value = preg_quote($value);

$tag_regex = “/<(".$tag.")[^>]*$attr\\s*=\\s*”.
“(['\\"])$value\\\\2[^>]*>(.*?)<\\/\\\\1>/”

preg_match_all($tag_regex,
$xml,
$matches,
PREG_PATTERN_ORDER);

return $matches[3];
}

4、匹配和解析email地址

代码示例（比较复杂些）：

function is_valid_email_address($email){
$qtext = ‘[^\\x0d\\x22\\x5c\\x80-\\xff]‘;
$dtext = ‘[^\\x0d\\x5b-\\x5d\\x80-\\xff]‘;
$atom = ‘[^\\x00-\\x20\\x22\\x28\\x29\\x2c\\x2e\\x3a-\\x3c'.
'\\x3e\\x40\\x5b-\\x5d\\x7f-\\xff]+’;
$quoted_pair = ‘\\x5c[\\x00-\\x7f]‘;
$domain_literal = “\\x5b($dtext|$quoted_pair)*\\x5d”;
$quoted_string = “\\x22($qtext|$quoted_pair)*\\x22″;
$domain_ref = $atom;
$sub_domain = “($domain_ref|$domain_literal)”;
$word = “($atom|$quoted_string)”;
$domain = “$sub_domain(\\x2e$sub_domain)*”;
$local_part = “$word(\\x2e$word)*”;
$addr_spec = “$local_part\\x40$domain”;

return preg_match(“!^$addr_spec$!”, $email) ? 1 : 0;
}

5、匹配URL

正则表达式：

{
\\b
# Match the leading part (proto://hostname, or just hostname)
(
# http://, or https:// leading part
(https?)://[-\\w]+(\\.\\w[-\\w]*)+
|
# or, try to find a hostname with more specific sub-expression
(?i: [a-z0-9] (?:[-a-z0-9]*[a-z0-9])? \\. )+ # sub domains
# Now ending .com, etc. For these, require lowercase
(?-i: com\\b
| edu\\b
| biz\\b
| gov\\b
| in(?:t|fo)\\b # .int or .info
| mil\\b
| net\\b
| org\\b
| [a-z][a-z]\\.[a-z][a-z]\\b # two-letter country code
)
)

# Allow an optional port number
( : \\d+ )?

# The rest of the URL is optional, and begins with /
(
/
# The rest are heuristics for what seems to work well
[^.!,?;"\\'<>()\[\]\{\}\s\x7F-\\xFF]*
(
[.!,?]+ [^.!,?;"\\'<>()\\[\\]\{\\}\s\\x7F-\\xFF]+
)*
)?
}ix

完整的文章请访问这里阅读（英文）

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