PHP Regular Expression Concepts

Delimiter

Usually use "/" as the delimiter to start and end, you can also use "#". Usually when there are many "/" characters in the string, use "#" as the delimiter, because the character needs to be escaped, such as a URI.

  <?php  /**   * delimiter   * **/ span> $regex   =   '/^http:\/\/([\w.]  +) \/([\w]+)/([\w]+)/([\w]+)/([\w]+]/([\w]+] \.html$/i ' ;  $str   =    ' http://php1234.cn/a/functions/2016/0905/50.html ' ;  $matches   =   Array   ();  if  ( preg_match  ( $regex ,  $str ,  $matches ))  { var_dump  ( $matches );  $regex   =   ' #^http:\/\/([\w.]  +) \/([\w]+)/([\w]+)/([\w]+)/([\w]+]/([\w]+) \.html$ #i ' ;  if  ( preg_match  ( $regex ,  $str ,  $matches ))  { var_dump  ( $matches ); } ?  

Output: Array (7) {  [0]=>  string (http://php1234.cn/a/functions/2016/0905/50.html) "  [1]=>  " String (Ten) "php1234.cn"  [2]=>  string (1) "A"  [3]=>  string (9) "Functions"  [4]=>  String (4) "  [5]=>  string (4)" 0905 "  [6]=>  string (2)" "}array (7) {  [0]=>  string ("http://php1234.cn/a/functions/2016/0905/50.html"  ) [1]=>  String (Ten) "php1234.cn"  [2]=>  string (1) "A"  [3]=>  string (9) "Functions"  [4]=>  String (4) "  [5]=> string (  4)" 0905 "  [6]=>  string (2)" 50 "}

"Modifier"

The symbol used to change the behavior of the regular expression, the last of the expressions in the above example,/I is a modifier to ignore the case, and a more commonly used is "X", which is used to denote ignoring spaces.

"Character Field"

The part that expands with square brackets is the word value field, as in the previous example: [\w].

"Qualifier"

Symbols such as [\w]{3,5} or [\w]*] or [\w]+ these [\w] represent qualifiers. {3,5} represents 3 to 5 characters. {3,} more than 3 characters, {, 5} up to 5. {3} three characters. * Represents 0 to more. + represents 1 to more.

"Off character"

^: In the word value field (e.g. [^\w]), the negation (not including the meaning) means "reverse selection".  before the expression, it begins with the current character. (/^n/i, which means start with N). Note: We often call "\" called "Skip character". Used to escape some special symbols, such as ".", "/"

"Wildcard character"

Determine the presence or absence of certain characters in a string! Format: Forward pre-check: (? =) corresponds to (?!) Negative meaning reverse pre-check: (? <=) corresponds to the (? <!) To express a negative meaning

<?php  /**   * Wildcard characters   * **/  $regex = '/(? <=c) d (? =e)/';  /* d followed by C, and D followed by e*/  $str = ' abcdefgk ';  $matches = Array ();  if (Preg_match ($regex, $str, $matches)) {      Var_dump ($matches);  }  $regex = '/(? <!c) d (?! e)/';//negative meaning  $str = ' abcdefgdk ';  if (Preg_match ($regex, $str, $matches)) {      Var_dump ($matches);  }?>

Output: Array (1) {  [0]=>  string (1) "D"}array (1) {  [0]=>  string (1) "D"}

"Lazy Match"

Format: Qualifier? Principle: "?" : The smallest data is used if there is a qualifier in front of it. such as "*" will take 0, and "+" will take 1, if it is {3,5} will take 3.

<?php  /**   * Lazy Matching   * **/  $regex = '/hel*/i ';$str = ' hellllllllllllllll ';if(Preg_match($regex,$str,$matches)){Var_dump($matches); }$regex = '/hel*?/i ';$str = ' hellllllllllllllll ';if(Preg_match($regex,$str,$matches)){Var_dump($matches); }$regex = '/hel+?/i ';$str = ' hellllllllllllllll ';if(Preg_match($regex,$str,$matches)){Var_dump($matches); }$regex = '/hel{5,8}?/i ';$str = ' hellllllllllllllll ';if(Preg_match($regex,$str,$matches)){Var_dump($matches); }?>

Output: Array (1) {  [0]=>  string (+) "Hellllllllllllllll"}array (1) {  [0]=>  string (2) "he"}array ( 1) {  [0]=>  string (3) "HeL"}array (1) {  [0]=>  string (7) "Helllll"}

*: 0 to multiple +:1 can also be written as {1,}? : 0 or 1 times.  

PHP Regular Expression Concepts