Question: I want to separate sentences by inserting a space between each period and letter but not between anything else like a dot and a bracket or a dot and a comma.

Consider this:

```none
This is a text.With some dots.Between words.(how lovely).

```

This probably has some solution in Perl or PHP but what I'm interested in is can it be done in a text editor that supports search/replace based on regexes? The problem is that it would match both the dot and the character and replace will completely obliterate both. In other words, is there a way to match "nothing" between those two characters?

Answer: 
You could use back references in the replace string. Typically it would look something like:

Search regex:

```
(\.)(\w)

```

Replacement pattern (notice the space):

```
$1 $2

```

The back references are stand-ins for the corresponding groups.

Alternatively, you could use lookarounds:

```
(?<=\.)(?=\w)

```

This doesn't "capture" the text, it would only match the position between the period and the letter/number (a zero-length string). Replacing it would, essentially, insert some text.

Really, though, it depends on the capabilities of your text editor. Very few text editors have a "complete" regular expression engine built-in. I use TextPad which has its own flavor of regular expression which largely does not support lookarounds (forcing me to use the first approach).
This segment of code solves your problem:

```
preg_replace('/([a-zA-Z]{1})\.([a-zA-Z]{1})/', '$1. $2', 'This is a text.With some dots.Between words.(how lovely).');

```

You should detect any character before and after dot and replace with blanco.