Remove a comma between two specific characters

It may be easier to do this in a few steps:

• Split on ~
• Transform the parts that are "inside" the ~ only
  • Simply replace ',' with ''
• Join the parts back together with ~

A regex solution

That said, it is possible to do this in regex, assuming an even number of ~:

<?php echo preg_replace( '/(^[^~]*~)|([^~]*$)|([^,~]*),|([^,~]*~[^~]*~)/', '$1$2$3$4', 'a,b,c,~d,e,f~,g,h,i,~j,k,l,~m,n,o~,q,r,~s,t,u' ); ?> 

The above prints (as seen on codepad.org):

a,b,c,~def~,g,h,i,~jkl~m,n,o~qr~s,t,u 

How it works

There are 4 cases:

• We're at the beginning of the string, "outside"
  • Just match until we find the first ~, so next time we'll be "inside"
  • So, (^[^~]*~)
• There are no more ~ till the end of the string
  • If there are even number of ~, we'll be "outside"
  • Just match until the end
  • So, ([^~]*$)
• If it's none of the above, we're "inside"
  • Keep finding the next comma before ~ (so we're still "inside")
    • So, ([^,~]*),
  • If we find ~ instead of a comma, then go out, then go back in on the next ~
    • So, ([^,~]*~[^~]*~)

In all case, we make sure we capture enough to reconstruct the string.

References

• regular-expressions.info/Character Classes, Anchors, Grouping and backreferences

$string = "Blah, blah, blah,~Part One, Part Two~,blah blah"; $pos1 = strpos($string, "~"); $substring = substr($string, $strpos, strlen($string)); $pos2 = strpos($string, "~"); $final = substr($substring, $pos1, $pos2); $replaced = str_replace(",", "", $final); $newString = str_replace($final, $replaced, $string); echo $newString; 

It does the job, but I wrote it right here and might have problems (at least performance problems).