Character Groups in PHP Regular Expressions

There are special commands that allow you to select entire groups of characters at once. The command \d means a digit from 0 to 9. The command \w denotes a digit, Latin letter, or underscore. The command \s denotes a space or whitespace character: space, newline, tab. You can invert the meaning of the command by writing a capital letter: for example, if \d is a digit, then \D is a non-digit.

Example

Let's find all digits:

php<?php
  $str = '1 12 123';
  $res = preg_replace('#\d#', '!', $str);
?>

As a result, the following will be written to the variable:

php'! !! !!!'

Example

Repetition operators consider group commands as a whole, meaning grouping parentheses are not needed. In the next example, the search pattern looks like this: digit from 0 to 9 one or more times:

php<?php
  $str = '1 12 123 abc @@@';
  $res = preg_replace('#\d+#', '!', $str);
?>

As a result, the following will be written to the variable:

php'! ! ! abc @@@'

Example

In the next example, the search pattern looks like this: anything one or more times, but not a digit from 0 to 9:

php<?php
  $str = '123abc3@@';
  $res = preg_replace('#\D+#', '!', $str);
?>

As a result, the following will be written to the variable:

php'123!3!'

Example

In this example, the search pattern looks like this: whitespace character once:

php<?php
  $str = '1 12 123 abc @@@';
  $res = preg_replace('#\s#', '!', $str);
?>

As a result, the following will be written to the variable:

php'1!12!123!abc!@@@'

Example

In this example, the search pattern looks like this: NON-whitespace character one or more times. All substrings separated by spaces will be replaced by '!':

php<?php
  $str = '1 12 123 abc @@@';
  $res = preg_replace('#\S+#', '!', $str);
?>

As a result, the following will be written to the variable:

php'! ! ! ! !'

Example

In this example, the search pattern looks like this: digit or letter one or more times. All substrings consisting of digits and letters will be replaced by '!':

php<?php
  $str = '1 12 123a Abc @@@';
  $res = preg_replace('#\w+#', '!', $str);
?>

As a result, the following will be written to the variable:

php'! ! ! ! @@@'

Example

In this example, the search pattern looks like this: NOT a digit and NOT a letter one or more times. In our case, this definition includes '@@@' and all spaces (they are also not digits and not letters). Pay attention to the fact that at the end there is one '!' - it was transformed from the string ' @@@' - with a space in front:

php$str = '1 12 123 Abc @@@';
$res = preg_replace('#\W+#', '!', $str);

As a result, the following will be written to the variable:

php'1!12!123!Abc!'

Practical Tasks

php<?php
  $str = 'a1a a2a a3a a4a a5a aba aca';
?>

php<?php
  $str = 'a1a a22a a333a a4444a a55555a aba aca';
?>

php<?php
  $str = 'aa a1a a22a a333a a4444a a55555a aba aca';
?>

php<?php
  $str = 'avb a1b a2b a3b a4b a5b abb acb';
?>

php<?php
  $str = 'ave a#b a2b a$b a4b a5b a-b acb';
?>

php<?php
  $str = 'ave a#a a2a a$a a4a a5a a-a aca';
?>