Question

Using Smarty template engine, but looking to pre-process HTML to change font-size across all CSS.

I want to make the whitespace(s) optional after the colon so it will match the number no matter what.

The issue is I can only seem to get optional matching as part of the expression, not the lookbehind.

RegEx: (?<=font-size: )[0-9]+

HTML excerpt:

body {
    font-family: 'Open Sans', sans-serif;
    font-size: 9pt; 
    height: 100%;
    min-height: 100%;
    display:block;    
}

Answer 1

Do not use lookbehinds when you can do it easily with a capturing group.

In your case you can do something like:

(\bfont-size:\s*)([0-9]+)

Then use the capturing groups $1 and $2 as you need.