Regex PCRE: skip the first n occurences?

Question

When using PCRE regex, is there a way to skip the first n occurences? I saw some examples with \K but I don't understand how to use it.

The goal is to find "px" values, so this is the regex I use:

(?!1px)(\d+)(px)

(?!1px) is used to ignore "1px".

Considering the following sample string, how could I skip the first (12px), or second (4px) match, to match only the third (2px)?

* {margin: 0; padding: 0; font-size: 12px; color: #555;}

.test {
    display: inline-block;
    border: 1px solid #000;
    box-shadow:  #aaa 4px 2px 6px;
    width: 36px;
    height: 24px;
}

Answer 1

You may use

(?s)^(?:.*?\b(?!1p)\d+px){2}.*?\K\b(?!1p)(\d+)(px)

See the regex demo

Details

• (?s) - DOTALL s flag that makes . match line break chars, too
• ^ - start of a string
• (?:.*?\b(?!1p)\d+px){2} - exactly two occurrences of any 0 or more chars, as few as possible, and then 1+ digits + px but 1px value
• .*? - any 0+ chars, as few as possible
• \K - match reset operator discarding the text matched so far
• \b - word boundary
• (?!1p) - no 1p allowed immediately to the right
• (\d+)(px) - 1+ digits captured in Group 1 and px captured in Group 2.