始终使用正则表达式在两个字符之间添加空格

Question

For those regex experts out there, can you help me tweak this regex to always add a whitespace between two matching characters.

My attempt is to resolve the interpolation security issue with Vue.js for data passed from the server side (Vue.js will try to evaluate anything between two curly braces). The goal is:

1. To always ensure whitespace between two curly braces
2. Not add additional whitespace where unnecessary.

My str_replace solution (which accomplishes goal #1 only)

str_replace(
    ['{',  '}',  '{',  '}',  '{',  '}' ],
    ['{ ', '} ', '{ ', '} ', '{ ', '} '],
    $value
);

My attempted regex thus far:

preg_replace('/({|}|{|}|{|})(\S)/', '$1 $2', $value);

So it checks for any matching character that isn't followed by whitespace and inserts whitespace between the two characters.

The regex works in most cases, but fails when you have three or more matching characters.

Ex: {{{ returns { {{ but the expected output would be { { {.

Answer 1

It doesn't work as you expect it because the first two { characters match the regex, the replacement is made then the search continues with the 3^rd character of the input string

You can solve this by turning the second parenthesis into a forward assertion. This way, the second } is not consumed on the first match and the next search starts with the second character of the input string:

preg_replace('/({|}|{|}|{|})(?=\S)/', '$1 ', $value);

There is only one capture group this way, $2 is always empty and it is not needed any more in the replacement string.

See it in action: https://3v4l.org/CaMHS. The +++ markers were added to show that the third { doesn't match the regex and no replacement occurs for it.