# \[X\] Backreference evaluation inside replacement string
**URL:** https://community.mp3tag.de/t/x-backreference-evaluation-inside-replacement-string/12111
**Category:** No Bugs
**Created:** [June 7, 2011, 3:15pm UTC](https://community.mp3tag.de/t/x-backreference-evaluation-inside-replacement-string/12111 "2011-06-07T15:15:07Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@ptrk.mj](https://community.mp3tag.de/u/ptrk.mj)
#### Post date: [June 7, 2011, 3:15pm UTC](https://community.mp3tag.de/t/x-backreference-evaluation-inside-replacement-string/12111/1 "2011-06-07T15:15:07Z")
When using regular expression inside title-formatting script (with $regexp() function) backreferences inside replacement string are evaluated after all scripting functions included in replacement are applied.
It makes performing replacements based on a content of backreferences impossible.
This doesn't apply to "Replace with regular expression" action! Action works as it's supposed to: first bacreferences are evaluated, then any existing scripting functions are applied.
Consider regular expression:
Input string: string
Regular expression: .+
Replacement: $if($eql($&,string),x,y)
Action applied to a field having "string" as content returns x while..
```
$regexp(string,string,$if($eql($&,string),x,y))
```
..returns y
---
### Author:  [@DetlevD](https://community.mp3tag.de/u/DetlevD)
#### Post date: [June 7, 2011, 3:49pm UTC](https://community.mp3tag.de/t/x-backreference-evaluation-inside-replacement-string/12111/2 "2011-06-07T15:49:12Z")
> [@ptrk.mj](#):
>
> ... Consider regular expression:
> Input string: string
> Regular expression: .+
> Replacement: $if($eql($&,string),x,y)
As fas as I know, nearly all of the Mp3tag scripting functions are not allowed to be used in the replacement part of the function $regexp.
Therefore your example is not a valid expression.
DD.20110607.1950.CEST
---
### Author:  [@ptrk.mj](https://community.mp3tag.de/u/ptrk.mj)
#### Post date: [June 7, 2011, 5:47pm UTC](https://community.mp3tag.de/t/x-backreference-evaluation-inside-replacement-string/12111/3 "2011-06-07T17:47:19Z")
> [@DetlevD](#):
>
> As fas as I know, nearly all of the Mp3tag scripting functions are not allowed to be used in the replacement part of the function $regexp.
What do you mean by "not allowed to"? They work! And I use them extensively.
---
### Author:  [@Florian](https://community.mp3tag.de/u/Florian)
#### Post date: [December 28, 2018, 2:30pm UTC](https://community.mp3tag.de/t/x-backreference-evaluation-inside-replacement-string/12111/4 "2018-12-28T14:30:28Z")
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