PHP – Regex OR (alternation) examples using pipe

Regex OR (alternation) in php can be matched using pipe (|) match character. This is very frequently used in regular expressions. Following are some ways pipe can be used in php regex

1. (hello|foo|bar) will match any of hello, foo, bar
2. (|foo|bar) will match foo or bar or nothing. It is equivalent to optional match (foo|bar)?
3. (^|foo) will match either foo or beginning of string.
4. (foo|$) will match either foo or end of string.

preg_match example for pattern (A|B|C)

PHP preg_match example which matches with hello or foo or bar with space in beginning and end (ignoring case)

<?php
if (preg_match('/ (hello|foo|bar) /i', "hello world. food bar and something", $matches, PREG_OFFSET_CAPTURE)) {
  echo "matched string:\n{$matches[0][0]} (at offset {$matches[0][1]})\n";
} else {
  echo "No match\n";
}
?>

matched string:
 bar  (at offset 17)

preg_replace example for pattern (^|A)

PHP preg_replace example which removes hello when preceded by space or is in beginning (ignoring case)

<?php
$str = "hello world. sayhello. hello world again";
$newstr = preg_replace('/(^| )hello/i', "", $str, -1, $count);
if ($count > 0) {
  echo "newstr after $count replacement(s):\n$newstr\n";
} else {
  echo "No replacement\n";
}
?>

newstr after 2 replacement(s):
 world. sayhello. world again

preg_replace example for pattern (A|$)

PHP preg_replace example which removes hello when followed by space or is in end (ignoring case)

<?php
$str = "hello world. hellos last hello";
$newstr = preg_replace('/hello( |$)/i', "", $str, -1, $count);
if ($count > 0) {
  echo "newstr after $count replacement(s):\n$newstr\n";
} else {
  echo "No replacement\n";
}
?>

newstr after 2 replacement(s):
world. hellos last 

Few points to note

1. It is better to use brackets when using regex OR.
2. Pipe can be escaped using backslash for its literal use (\|).