Extracting Substrings

A substring is a string within a string. For example, tab and able are both substrings of table. You can use the[ ] operator to specify a substring using this syntax:

string[ [ start, ] length ]

string is the string containing the substring.

start is a number specifying where the substring starts. The first character of string counts as 1. If start is 0 or a negative number, the starting position is assumed to be 1. If start is omitted, the starting position is calculated according to the following formula:

string.length - substring.length + 1

Trailing Substrings. You can specify a trailing substring by omitting start from the syntax. For example, this specification:
```
"1234567890" [5]
```
returns the substring:
```
67890
```
Delimited Substrings. You can extract a delimited substring using this syntax:
```
string [ delimiter, instance, fields ]
```
- string is the string containing the substring.
- delimiter specifies the character that delimits the substring.
- instance specifies the instance of delimiter where the extraction is to start.
- fields specifies the number of fields to extract.
  The delimiters that mark the start and end of the extraction are not returned, but if you extract more than one string, any interim delimiters are returned. This syntax works the same way as the Field function.
Assigning a Substring to a Variable. All substring syntaxes can be used with the = operator to replace the value normally returned by the [ ] operator with the value assigned to the variable. For example:
```
A="12345"
A[3]=1212
```
returns the result 121212.

This syntax works the same way as the FieldStore function.