preg_replace("/\[b\](.*)\[\/b\]/Usi", "<strong>$1</strong>", "Some text here... [b][b]Hello, [b]PHP![/b][/b][/b] ... [b]and here[/b]");
returns
Some text here... <strong>[b]Hello, [b]PHP!</strong>[/b][/b] ... <strong>and here</strong>
But i need to replace all [b]...[/b] tags. Why this doesn't happen in my case?
Yes, a multi-pass approach is required if the elements are nested. This can be accomplished in one of two ways; matching from the inside out or from the outside in. Here are two tested scripts with fully commented regexes which illustrate each technique:
<?php // test.php Rev:20121016_0900
$re = '% # Match innermost [b]...[/b] structure.
\[b\] # Literal start tag.
( # $1: Element contents.
# Use Friedls "Unrolling-the-Loop" technique:
# Begin: {normal* (special normal*)*} construct.
[^[]* # {normal*} Zero or more non-"[".
(?: # Begin {(special normal*)*}.
\[ # {special} Tag open literal char,
(?!/?b\]) # but only if NOT [b] or [/b].
[^[]* # More {normal*}.
)* # Finish {(special normal*)*}.
) # $1: Element contents.
\[/b\] # Literal end tag.
%x';
printf("Replace matching tags from the inside out:\n");
$text = file_get_contents('testdata.txt');
$i=0; // Keep track of iteration number.
printf("i[%d]=%s", $i++, $text);
while(preg_match($re, $text)){
$text = preg_replace($re, '<strong>$1</strong>', $text);
printf("i[%d]=%s", $i++, $text);
}
?>
'''
Replace matching tags from the inside out:
i[0]=Some text here... [b][b]Hello, [b]PHP![/b][/b][/b] ... [b]and here[/b]
i[1]=Some text here... [b][b]Hello, <strong>PHP!</strong>[/b][/b] ... <strong>and here</strong>
i[2]=Some text here... [b]<strong>Hello, <strong>PHP!</strong></strong>[/b] ... <strong>and here</strong>
i[3]=Some text here... <strong><strong>Hello, <strong>PHP!</strong></strong></strong> ... <strong>and here</strong>
'''
<?php // test.php Rev:20121016_0901
$re = '% # Match outermost [b]...[/b] structure.
\[b\] # Literal start tag.
( # $1: Element contents.
(?: # Zero or more contents alternatives.
[^[]* # Either non-[b]...[/b] stuff...
(?: # Begin {(special normal*)*}.
\[ # {special} Tag open literal char,
(?!/?b\]) # but only if NOT [b] or [/b].
[^[]* # More {normal*}.
)* # Finish {(special normal*)*}.
| (?R) # Or a nested [b]...[/b] structure.
)* # Zero or more contents alternatives.
) # $1: Element contents.
\[/b\] # Literal end tag.
%x';
printf("Replace matching tags from the outside in:\n");
$text = file_get_contents('testdata.txt');
$i=0; // Keep track of iteration number.
printf("i[%d]=%s", $i++, $text);
while(preg_match($re, $text)){
$text = preg_replace($re, '<strong>$1</strong>', $text);
printf("i[%d]=%s", $i++, $text);
}
?>
'''
Replace matching tags from the outside in:
i[0]=Some text here... [b][b]Hello, [b]PHP![/b][/b][/b] ... [b]and here[/b]
i[1]=Some text here... <strong>[b]Hello, [b]PHP![/b][/b]</strong> ... <strong>and here</strong>
i[2]=Some text here... <strong><strong>Hello, [b]PHP![/b]</strong></strong> ... <strong>and here</strong>
i[3]=Some text here... <strong><strong>Hello, <strong>PHP!</strong></strong></strong> ... <strong>and here</strong>
'''
Note the (?R) recursive expression used in the second approach.
The reason it doesn't work: You catch the first [b], then move on to the next [/b], and leave anything in between unchanged. Ie, you change the outer [b] tags, but not the ones nested inside.
Your comment to @meza suggests you want to replace the pseudo tags in pairs, or else leave them untouched. The best way to do this is to use multiple passes, like this
$markup = "Some text here... [b][b]Hello, [b]PHP![/b][/b][/b] ... [b]and here[/b]";
$count = 0;
do {
$markup = preg_replace("/\[b\](.*?)\[\/b\]/usi", "<strong>$1</strong>", $markup, -1, $count );
} while ( $count > 0 );
print $markup;
I'm not even sure if you can do it in a one-line regex, but even if you could, it would be rather complex and therefore hard to maintain.
Why use regex for this particular case? You could get away with a simple string replace every [b] to strong and every [/b] to the /strong.
