PHP正则表达式加号

Question

I'm playing around with PHP Regex in order to improve my skills with it.

I'm having a hard time trying to understand the plus sign - so I wrote the following code:

$subject = 'aaa bbb cccc dddd';
echo preg_replace('/(\w)/',"$1*",$subject) . '<br>';
echo preg_replace('/(\w+)/',"$1*",$subject) . '<br>';
echo preg_replace('/(\w)+/',"$1*",$subject) . '<br>';

With results in:

a*a*a* b*b*b* c*c*c*c* d*d*d*d*
aaa* bbb* cccc* dddd*
a* b* c* d*

I don't understand why these results come about. Can someone please explain what's going on in this example

Answer 1

in regular expressions, + means one or more of the preceding character or group .

The pattern /(\\w)/ , means match a single word character ( a-zA-Z0-9_ ) in a single group. So it will match each letter. The first match group will be just a . The replace will replace each individual letter with that letter followed by an asterisk.

The pattern /(\\w+)/ will match one or more word characters in a group. So it will match each block of letters. The first match group will be aaa . The replace will replace each block of multiple letters followed by a asterisk.

The last pattern /(\\w)+/ is a little more tricky, but will match a single word character in a group but the + means that it will match one or more of the group. So the first match will be a , but the replace will replace all of the groups until there isn't a match with the last matched group (of course followed by an asterisk). So if you tried the string aaab ccc , your result would end up as b* c* . b is the last matched group in the first sequence and so the replace would use that.

Answer 2

Your mistake isn't the plus sign, it's understanding what the parentesis is for and how it works. The parenthesis is for grouping your match into a variable, hence why you can do $1, the second set of () gives you $2 and so on...

• (\\w) means 1 word character
• (\\w+) means 1 or more word characters
• (\\w)+ matches 1 or more word characters, but only the first one is put into the variable, because only the \\w is inside the paranthesis