CODEWITHSUNDEEP
phpregex
FarCreek
FarCreek

Reputation: 47

Matching math between < > symbols

I'm trying to build a function that matches the math expression between two greater (or equal) or smaller (or equal) symbols.

I have the following preg_match function:

preg_match("/(<=?|>=?)(([0-9]|\+|\(|\))+)(<=?|>=?)/", "2<(2+2)<8", $matches);

When I read the $matches array I get:

Array
(
    [0] => <(2+2)<
    [1] => <
    [2] => (2+2)
    [3] => )
    [4] => <
)

Can anyone explain why the closing ) gets matched as part of the (2+2) and on it's own? I would like it to only match the whole (2+2).

Upvotes: 0

Views: 72

Answers (2)

user557597
user557597

Reputation:

Because you have a quantified capture group (...)+

Each pass through the capture group, resets the capture group to empty.
The result is you only see the last capture.

You can see it below as 3 start/end. 

 ( <=? | >=? )                 # (1)
 (                             # (2 start)
      (                             # (3 start)
           [0-9] 
        |  \+
        |  \(
        |  \) 
      )+                            # (3 end)
 )                             # (2 end)
 ( <=? | >=? )                 # (4)

The individual pieces are of no use in this case,
changing it to a cluster group will exclude it from the output array. 

 ( <=? | >=? )                 # (1)
 (                             # (2 start)
      (?:
           [0-9] 
        |  \+
        |  \(
        |  \) 
      )+
 )                             # (2 end)
 ( <=? | >=? )                 # (3)

Output 

 **  Grp 0 -  ( pos 0 , len 7 ) 
<(2+2)<  
 **  Grp 1 -  ( pos 0 , len 1 ) 
<  
 **  Grp 2 -  ( pos 1 , len 5 ) 
(2+2)  
 **  Grp 3 -  ( pos 6 , len 1 ) 
<

Upvotes: 0

Dmitry Egorov
Dmitry Egorov

Reputation: 9650

Because you've got two capturing groups for the expression between comparison signs:

(<=?|>=?)(([0-9]|\+|\(|\))+)(<=?|>=?)
         ^^              ^ ^
         |`----- $3 -----' |
         `------- $2 ------'

Change it to

(<=?|>=?)((?:[0-9]|\+|\(|\))+)(<=?|>=?)
           ^^

Upvotes: 1

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