PHP中的正则表达式：替换字符串之间的文本

Question

Okay I have made some progress on a problem I am solving, but need some help with a small glitch.

I need to remove all characters from the filenames in the specific path images/prices/ BEFORE the first digit, except for where there is from_, in which case remove all characters from the filename BEFORE from_.

Examples:

BEFORE                                AFTER
images/prices/abcde40.gif           > images/prices/40.gif
images/prices/UgfVe5559.gif         > images/prices/5559.gif
images/prices/wedsxcdfrom_88457.gif > images/prices/from_88457.gif

What I've done:

$pattern = '%images/(.+?)/([^0-9]+?)(from_|)([0-9]+?)\.gif%';
$replace = 'images/\\1/\\3\\4.gif';
$string = "AAA images/prices/abcde40.gif BBB images/prices/wedsxcdfrom_88457.gif CCC images/prices/UgfVe5559.gif DDD";
$newstring = str_ireplace('from_','733694521548',$string);
while(preg_match($pattern,$newstring)){
    $newstring=preg_replace($pattern,$replace,$newstring);
}
$newstring=str_ireplace('733694521548','from_',$newstring);
echo "Original:
$string

New:
$newstring";

My expected output is:

AAA images/prices/40.gif BBB images/prices/from_88457.gif CCC images/prices/5559.gif DDD"

But instead I am getting:

AAA images/prices/40.gif BBB images/from_88457.gif CCC images/5559.gif DDD

The prices/ part of the path is missing from the last two paths.

Note that the AAA, BBB etc. portions are just placeholders. In reality the paths are scattered all across a raw HTML file parsed into a string, so we cannot rely on any pattern in between occurrences of the text to be replaced.

Also, I know the method I am using of substituting from_ is hacky, but this is purely for a local file operation and not for a production server, so I am okay with it. However if there is a better way, I am all ears!

Thanks for any assistance.

Answer 1

You can use lookaround assertions:

preg_replace('~(?<=/)(?:([a-z]+)(?=\d+\.gif)|(\w+)(?=from_))~i', '', $value);

Explanation:

(?<=/)          # If preceded by a '/':
(?:             # Begin group
 ([a-z]+)       #   Match alphabets from a-z, one or more times
 (?=\d+\.gif)   #   If followed followed by digit(s) and '.gif'
 |              #   OR
 (\w+)          #   Match word characters, one or more times
 (?=from_)      #   If followed by 'from_'
)               # End group

Visualization:

Code:

$pattern = '~(?<=/)(?:([a-z]+)(?=\d+\.gif)|(\w+)(?=from_))~i';
echo preg_replace($pattern, '', $string);

Demo