[SOLVED] need help understanding what this expression does

[jake2891]

Hi I have been going through documentation on regular expressions and still dont fully understand what this expression is doing. can someone please explain it for me. thanks

 

$str = preg_replace('/^([ ]+<[^>]+>.+)<\/[^>]+>$/', '$1', $str);

[nrg_alpha]

The basic rundown is:

 

^ from the beginning of string

( start a new capture

[ ]+ a character class that looks to see if the current character is a space (one or more times, due to the +)

<[^>]+> followed by <, then anything that is not > (due to the [^>]) one or more times, then a >

.+ the dot is a dot_match_all wildcard character that matches anything that is not a newline by default (one or more times).

) close capture

<\/[^>]+> followed by <, then / (this slash needs to be escaped using \ before it, as the delimiters surrounding the entire pattern is the / character), then anything that is not a > (one or more times), then finally >

$ end of string.

 

So anything that is within the set of parenthesis that the pattern matches gets stored into a regex variable called $1 (which is used as the second parameter in the regex as a replacement in $str.

 

There are some bad forms with this though.. by example, if you look for a single character space (as in the space within the first character class [ ]), you don't need to encase it in a character class.. simply providing a literal space (or the hex vale \x20). Other aspects like .+ means its greedy.. match anything up to a newline. But as a result, the regex engine will now start to backtrack to accommodate what comes after that in the pattern (which can create accuracy / speed problems).

 

you can learn more about regex by viewing the following links:

 

regex tutorials

weblogtools

phpfreaks regex resources

phpfreaks regex tutorial

[jake2891]

thanks for taking the time to clear things up for me. that makes alot more sense now.