# using regex to replace values of a certain variable  all files in a folder

**What is a Regular Expression?**

On an abstract level a regular expression, regex for short, is a shorthand representation for a set. A set of strings.

**What are Regular Expressions used for?**

Regular expressions are useful in any scenario that benefits from full or partial pattern matches on strings. These are some common use cases:
- verify the structure of strings
- extract substrings form structured strings
- search / replace / rearrange parts of the string
- split a string into tokens

**Scenario**

Let's assume you want to remove all the occurrences of the variable xyz in all files in a certain directory, you would use [grepWin](https://sourceforge.net/projects/grepwin/files/latest/download) software to perform the regex search on the entire directory.

For instance, to replace below occurrence of variable (there is a blank line after the value abc)
```
xyz:
  - abc
    
```

you will need to use 
```
xyz:[\n\r](\s\s-.)([^\s]+)[\n\r]
```



to replace below occurrence of variable (no blank line after value abc)

```
xyz:
  - abc
```
you will need to use 
```
xyz:[\n\r](\s\s-.)([^\s]+)
```


to replace below occurrence of variable 
```
xyz: abc
```
you will need to use
```
xyz:(\s.)([^\s]+)
```