Regex Game - Replace each word except specific ones by a variable number of characters

Question

Hey there, you Regex Lovers ! I'm quite in Regex, these times and had a purely theorical problem. To put it simple, I will present it as a game. The game : Let's say you have a list of words separated by spaces. What I call a word is as they are defined by regular expressions : `[a-zA-Z_0-9]+` (There is no empty word here) Example of list : `Horse Banana Joker RoXx0r A_Long_Word Joker 1337` What I want you to do is replace each word except Joker by a number of $ equal to the number of character of the matched word. With our previous list we would obtain : `$$$$$ $$$$$$ Joker $$$$$$ $$$$$$$$$$$ Joker $$$$` In fewer words : I want a regex that matches each character that does not belong to the word "Joker" (In the string, I mean, not that compose the word Joker) While it is not easy, it's not impossible (I have my own regex for that). That's why I will set some rules. The rules : - It must be done with only 1 regex - I will not accept any regex that works only in specific languages - I…

Accepted Answer

For PCRE/Perl/Ruby/Java/.net

find:

```
(?:\G(?!^)|(?<!\S)(?!Joker(?!\S)))\S
```

replace:

```
$
```

demo

pattern details:

```
(?:
    \G (?!^) # contigous to a previous match (but not at the start of the string)
  |        # OR
    (?<!\S)  # not preceded by a non white-space
    (?!Joker(?!\S)) # not followed by the forbidden word
)
\S   # a non-whitespace character
```

If your words are only composed of word characters, you can simplify the pattern playing with word and non-word boundaries: `(?:\G\B|\b(?!Joker\b))\w`

Other way (PCRE/Perl): without the `\G` feature and with the backtracking control verb `(*SKIP)` (need less steps):

```
\s*(?:Joker(?:\s+|$))*(*SKIP)\K.
```

To be clear `(*SKIP)` is only useful when the string ends with the forbidden word or a whitespace. You can also replace it with `(*COMMIT)`.

demo

or:

```
\bJoker\b(*SKIP)(*F)|\S
```

and with pypi python regex module (that has a word boundary for the start and one for the end of a word):

```
\mJoker\M(*SKIP)(*F)|\S
```

A one that works with Javascript (if there's something to replace only):

find:

```
((?:\s+|\bJoker\b)*)\S((?:\s+Joker)*\s*$)?
```

replace: (backreference to group1, escaped $, backreference to group2)

```
$1$$$2 
```

demo

An other Javascript version that uses the y flag (that forces the matches to be contigous), but unfortunately this one isn't supported by Internet Explorer, Safari and mobile browsers except Firefox mobile:

```
var strs = ['Horse Banana Joker RoXx0r A_Long_Word Joker 1337 Joke Poker Joker', 'Joker Joker Joker'];

strs.forEach(function (s) {
    console.log(s.replace(/(?=((?:\s+|\bJoker\b)*))\1./gy, '$1$$'));
});
```

The `(?=(...))\1` emulates an atomic group (that forbids backtracking).

Regex Game - Replace each word except specific ones by a variable number of characters

Solution

Problem