How to use a variable as a className inside preg_match_all parameters?

Note: this question is not related to WordPress, but rather to PHP directly and should be asked at StackOverflow but I have provided a very basic answer below to help you on your way. However be aware that this answer is not necessarily the best way to go about solving your problem, nor is it meant to be efficient.

// assuming get_the_content() equates to ↓

$content="<p class="1">P1 Content</p><p class="2">P2 Content</p><p class="3">P3 Content</p>";

$classNames = ['1', '2', '3'];

$matches = [];

foreach( $classNames as $className ) {

    $pattern = sprintf('\<p\sclass=\"%s\"\>(.*?)\<\/p\>', $className);

    preg_match("/{$pattern}/i", $content, $found); 

    $matches[] = $found;

}

var_dump($matches);

/*
array(3) {
  [0]=>
  array(2) {
    [0]=>
    string(27) "<p class="1">P1 Content</p>"
    [1]=>
    string(10) "P1 Content"
  }
  [1]=>
  array(2) {
    [0]=>
    string(27) "<p class="2">P2 Content</p>"
    [1]=>
    string(10) "P2 Content"
  }
  [2]=>
  array(2) {
    [0]=>
    string(27) "<p class="3">P3 Content</p>"
    [1]=>
    string(10) "P3 Content"
  }
}
*/

UPDATE 1:

If you do not have a fixed, known-list (ahead of time) of class names, then use the following example which utilises preg_match_all and looks for any <P> tag with class = NUMBER.

<p class="1">Content</p>
<p class="2">Content</p>
<!-- etc.. -->
<p class="234">Content</p>

$content="<p class="1">P1 Content</p><p class="2">P2 Content</p><p class="3">P3 Content</p>";

$pattern = sprintf('\<p\sclass=\"%s\"\>(.*?)\<\/p\>', '[0-9]+');

var_dump($pattern);

preg_match_all("/{$pattern}/i", $content, $found);

var_dump($found);

/*
array(2) {
  [0]=>
  array(3) {
    [0]=>
    string(27) "<p class="1">P1 Content</p>"
    [1]=>
    string(27) "<p class="2">P2 Content</p>"
    [2]=>
    string(27) "<p class="3">P3 Content</p>"
  }
  [1]=>
  array(3) {
    [0]=>
    string(10) "P1 Content"
    [1]=>
    string(10) "P2 Content"
    [2]=>
    string(10) "P3 Content"
  }
}
*/

/**
 * Iterate over the results (if any) and do something with the content.
 */
if ( is_array($found) && isset($found[1]) ) {

    foreach( $found[1] as $content ) {

        echo $content;

    }

}

Results in:

P1 Content
P2 Content
P3 Content

I advise that you prefix your class names such as:

<p class="myprefix-1">Content</p>
<p class="myprefix-2">Content</p>
<!-- etc.. -->
<p class="myprefix-234">Content</p>

If so, make sure to update your regex pattern:

$pattern = sprintf('\<p\sclass=\"myprefix-%s\"\>(.*?)\<\/p\>', '[0-9]+');

REPL.IT DEMO

Related Posts:

  1. OR condition in Regex
  2. Using or ‘|’ in regex [duplicate]
  3. What is a Context Free Grammar?
  4. What does \’.- mean in a Regular Expression
  5. How can I exclude one word with grep?
  6. How can I validate an email address using a regular expression?
  7. Regular expression to match a line that doesn’t contain a word
  8. Difference between \b and \B in regex
  9. Regex: ignore case sensitivity
  10. Python regex AttributeError: ‘NoneType’ object has no attribute ‘group’
  11. Regular Expression for alphanumeric and underscores
  12. What is the difference between the regular expressions [^\d\s] and [\D\S]
  13. Regular Expression for alphanumeric and underscores
  14. grep –ignore-case –only
  15. Regex AND operator
  16. python re.split() to split by spaces, commas, and periods, but not in cases like 1,000 or 1.50
  17. python re.split() to split by spaces, commas, and periods, but not in cases like 1,000 or 1.50
  18. Regex to test if string begins with http:// or https://
  19. Regular expression negative lookahead
  20. Regular Expression to match string starting with a specific word
  21. Regular Expressions: Is there an AND operator?
  22. Regular expression to match standard 10 digit phone number
  23. Regular Expression.how to add optional character at end of regex
  24. Regular Expression with wildcards to match any character
  25. What is a non-capturing group in regular expressions?
  26. How to validate an email address in PHP
  27. What is a non-capturing group in regular expressions?
  28. Regular expression to match standard 10 digit phone number
  29. How to use Regular Expressions (Regex) in Microsoft Excel both in-cell and loops
  30. Python string.replace regular expression
  31. How to use Regular Expressions (Regex) in Microsoft Excel both in-cell and loops
  32. Python string.replace regular expression
  33. Symbol for any number of any characters in regex?
  34. Grep and Python
  35. Regex – Does not contain certain Characters
  36. How to replace ” \ ” with ” \\ ” in java
  37. A regular expression to exclude a word/string
  38. Regex how to match an optional character
  39. How to validate an email address in JavaScript
  40. Using Bash regex match (=~) where regex includes quotes (” characters)
  41. Regular expression to match a word or its prefix
  42. How can I remove punctuation from input text in Java?
  43. Regular expression for exact match of a string
  44. Using the star sign in grep
  45. Regex optional group
  46. Regex for string contains?
  47. Regular Expression – Validate Gmail addresses
  48. Java; String replace (using regular expressions)?
  49. How can I remove punctuation from input text in Java?
  50. How can I output only captured groups with sed?
  51. How to negate specific word in regex?
  52. What is the grep equivalent in Python?
  53. javascript HTML from document.body.innerHTML
  54. Which regular expression operator means ‘Don’t’ match this character?
  55. Python re.split() vs split()
  56. How can I write a regex which matches non greedy?
  57. Negative matching using grep (match lines that do not contain foo)
  58. How to exclude a specific string constant?
  59. How to replace a whole line with sed?
  60. How to validate phone number using PHP?
  61. Validate phone number with JavaScript
  62. Can a URL contain a semicolon and still be valid?
  63. How is \\n and \\\n interpreted by the expanded regular expression?
  64. Regex to match only uppercase “words” with some exceptions
  65. Regular Expression to reformat a US phone number in Javascript
  66. Regex to match only letters
  67. Finding the complement of a DFA?
  68. RegEx to find two or more consecutive chars
  69. Regex to match an optional ‘+’ symbol followed by any number of digits
  70. Regex Match all characters between two strings
  71. Regular expression to match a word or its prefix
  72. Regular expressions in C: examples?
  73. How to ignore whitespace in a regular expression subject string?
  74. Regex: matching up to the first occurrence of a character
  75. Regex credit card number tests
  76. Regular expression – starting and ending with a character string
  77. Regular expression to stop at first match
  78. /bb|[^b]{2}/ how does it work?
  79. How do I match any character across multiple lines in a regular expression?
  80. Regex that matches integers in between whitespace or start/end of string only
  81. Regular Expressions- Match Anything
  82. What is the difference between square brackets and parentheses in a regex?
  83. Regex for “AND NOT” operation
  84. Escaping Parentheses in Regex
  85. How to replace a string in an existing file in Perl
  86. What is the best regular expression to check if a string is a valid URL?
  87. Regex empty string or email
  88. Regex difference: (\w+)? and (\w*)
  89. Wrap element with in any number of elements with PHP
  90. extract shortcodes from string
  91. preg_replace and comment_form_defaults
  92. 404 regular-expression advice needed
  93. How to set up redirects for category paginated pages after adding /category/ to permalink
  94. Adding span tags to post titles using regex
  95. Remove Custom HTML Posts Using Regex
  96. Regexes allow only [a-z][A-Z][0-9] and –
  97. Call Shortcode Attribute Value within another function
  98. How to fix old embeded iframes with WP-CLI search-replace
  99. Regex For Extracting First 4 letters of search term
  100. Use Regex Redirection via htsaccess for RSS Feeds WordPress