# RegEx and Back References
**URL:** https://community.mp3tag.de/t/regex-and-back-references/4880
**Category:** Support
**Created:** [May 15, 2007, 2:39pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880 "2007-05-15T14:39:15Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@JFord](https://community.mp3tag.de/u/JFord)
#### Post date: [May 15, 2007, 2:39pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880/1 "2007-05-15T14:39:15Z")
I am a novice at regular expressions, and MP3Tag is where I am cutting my teeth. I still find that I am wrong many more times than I am right and I don't want to use this forum to teach me how to create them. I am hoping that I have found an issue and it's not my misunderstanding.
I am attempting to parse a Comment field with the text "Before T3 After". However "T3" may be T1, T2, T3, T4, or T5. I figure that in RegEx the way you best describe that is "T[1-5]". Assuming I am correct I would then think that
> $RegExp(%Comment%,^(._)T([1-5])(._)$,$1/$2/$3)
would return
> "Before /3/ After"
but instead I get
> "Before //3 After"
QUESTION #1: Why didn't that example work? Do character classes not return references?
During the next hour as I throw anything I can find at this problem (and there are so many things in RegEx) I try
> $RegExp(%Comment%,^(._)T([1-5]+)(._)$,$1/$2/$3)
which I still think should work (but I still might be wrong about that), it returns an error (or two):
> REGEXP ERROR: Regular expression
> Invalid preceding regular expression
QUESTION #2: Why did that expression return an error?
Finally, I came up with
> $RegExp(%Comment%,^(._)T(\d)(._)$,$1/$2/$3)
which works well enough for this time (since there will never be a T0 or T6-T9, but I don't care if there is then matching to it is OK)
QUESTION #3: Is there a better way to do what I want restricting to T[1-5]?
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### Author:  [@dano](https://community.mp3tag.de/u/dano)
#### Post date: [May 15, 2007, 3:03pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880/2 "2007-05-15T15:03:55Z")
You must write '[' and ']' when you use scripting.
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### Author:  [@JFord](https://community.mp3tag.de/u/JFord)
#### Post date: [May 15, 2007, 4:05pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880/3 "2007-05-15T16:05:07Z")
I don't understand.
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### Author:  [@dano](https://community.mp3tag.de/u/dano)
#### Post date: [May 15, 2007, 8:02pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880/4 "2007-05-15T20:02:54Z")
$RegExp(%Comment%,^(._)T('['1-5']')(._)$,$1/$2/$3)
---
### Author:  [@JFord](https://community.mp3tag.de/u/JFord)
#### Post date: [October 29, 2007, 11:32pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880/5 "2007-10-29T23:32:56Z")
I am back, and now I believe I might have the opposite problem. This time I need to find the square bracket literals ("[" and]") in a string (actually isolate what's between square brackets). So, I would think that
- $regexp(%Title%,^(._)[(._)],$2)
would do the trick, but that just returns an error. I assume it's something funny about the square bracket character again. Is that right?
Also, are their any other differences between vanilla Regular Expressions and the implementation within the $RegExp function?
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### Author:  [@Florian](https://community.mp3tag.de/u/Florian)
#### Post date: [November 4, 2007, 7:10pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880/6 "2007-11-04T19:10:04Z")
The problem is that we are using two different languages within the $regexp function - title formatting language and regular expressions. Unfortunately, I have no means of seamless language composition at the moment.
The square brackets are special characters in the title formatting language and you have to quote them with ' if you want them to be displayed. As a consequence the call to the $regexp function should look like this
$regexp(%Title%,^(.\*)\'['(.\*)\']',$2)
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### Author:  [@JFord](https://community.mp3tag.de/u/JFord)
#### Post date: [November 7, 2007, 3:41pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880/7 "2007-11-07T15:41:31Z")
I see how it works! Thanks, that finally makes sense:
'[' starts a character class definition
'[' is the literal open square bracket character
I actually solved the problem using escape sequences (\x5b for '[' and \x5d for ']')
$regexp(%Title%,^(.+)(\x5b)(.+)(\x5d),$3)
Is the square bracket the only issue within the RegExp function?
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### Author:  [@Florian](https://community.mp3tag.de/u/Florian)
#### Post date: [November 7, 2007, 6:13pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880/8 "2007-11-07T18:13:31Z")
> [@JFord](#):
>
> Is the square bracket the only issue within the RegExp function?
To my current knowledge: yes.
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### Author:  [@system](https://community.mp3tag.de/u/system)
#### Post date: [February 9, 2026, 12:20pm UTC](https://community.mp3tag.de/t/regex-and-back-references/4880/9 "2026-02-09T12:20:17Z")