# Regular Expressions
**URL:** https://community.mp3tag.de/t/regular-expressions/521
**Category:** Howto
**Created:** [October 16, 2003, 4:47pm UTC](https://community.mp3tag.de/t/regular-expressions/521 "2003-10-16T16:47:29Z")
**Posts on this page:** 1
**Showing post:** 71
### Author:  [@vkostas](https://community.mp3tag.de/u/vkostas)
#### Post date: [December 12, 2020, 11:54am UTC](https://community.mp3tag.de/t/regular-expressions/521/71 "2020-12-12T11:54:34Z")
Few tips can help reduce complexity of Regular Expressions and likely make them run faster (see point 5).
Mp3Tag uses Perl Regular Expressions.
1. **`$`** vs **`\`** _in replacement section_.
e.g. swap artist name. Mercury, Freddy --\> Freddy Mercury
RegExp: **`(\w+),(\w+)`**
ReplaceWith: **`\2, \1`**
**`\2, \1`** is equivalent to **`$2, $1`** and **`${2} ${1}`**
The later is very useful if a fixed digit must be added after a group, e.g. **`${1}3`** adds digit 3 after group1.
**`\w+`** captures a series of ASCII characters.
2. _Named group_.
Ever wondering creating a complex regex and then starting counting the groups (1,2,3)? What if you create or remove a group? use **?\<...\>** to give a human readable name to your group. We create here two groups: **`first`** and **`last`**.
Above regexp would become:
RegExp: **`(?\w+),(?\w+)`**
ReplaceWith: **`$+{first} $+{last}`**
Note the additional **`+`** here, **`$+{first}`** which is not used in numbered groups **`$1`** , **`${1}`**.
3. _Reuse pattern_.
Named group can be used for recurring patterns. Could make your regex more human readable.
if we create a group **name** for our first/last name pattern **`(?\[^\s]+)`** we can reuse it.
RegExp: **`(?(?\w+)), (?(?&name))`**
ReplaceWith: **`$+{first} $+{last}`**
Note that **`(?&name)`** only reuses the pattern **`name`**. Their actual captured values can be anything. The three groups **`last`** , **`name`** and **`first`**.
4. Reuse pattern relative position
RegExp: **`(?(?\w+)), (?(?-2))`**
ReplaceWith: **`$+{first} $+{last}`**
**`((?-2))`** here goes two groups behind (the 1st is its wrapper and the 2nd is the group **`name`** ).
5. _Reuse value_. **`\k{name}`**
To capture same value use **`\k{name}`** , instead of **`(?&name)`**. This is the similar to what we normally do with **`\1`**.
6. _LookBehind_. _Magic switch_ **`\K`**.
Perl supports only fixed length look behind (this is a pattern we want to check before the current cursor) but it since some versions includes the magic switch **`\K`**.This switch tells the system to use only the part after this switch.
Example, delete trailing non-ASCII characters of our above pattern.
RegExp: **`(?(?\w+))\s(?(?&name))\K[^\w]+`**
ReplaceWith **`""`**
Anything before the **`\K`** is not involved in the replacement part.
All above are 100% tested in:
- Actions (`replace with regular expression`)
Enter source tag (e.g. artist) in **Field:**.
Enter text **RegExp** into **Regular Expression:**.
Enter text **ReplaceWith** into **Replaces Matches with:**.
- Function `$regexp(Text,'Find','Replace')`
Enter source tag (e.g. %artist%) in **Text**.
Enter text **RegExp** into **Find**.
Enter text **ReplaceWith** into **Replace**.
- WebSources `regexpreplace "Find" "Replace"`
Enter text **RegExp** into **Find**.
Enter text **ReplaceWith** into **Replace**.
Happy coding & testing.
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_[View the full topic](https://community.mp3tag.de/t/regular-expressions/521)._