Regular Expression for Excluding Special Characters

How to use lookahead to exclude special characters in a regular expression

The (.*\d+x\d+)*^(?![\+]+\d\+\d) pattern matches and captures into Group 1 an optional sequence of any zero or more chars other than line break chars, as many as possible, then 1+ digits, x, 1+ digits, and then requires the start of string position (this makes the pattern fail all the time!) and makes sure there is no one or more + chars, a digit, a + and a digit immediately to the right of the current location. This looks like a very corrupt pattern that never matches any string.

Your logic can be implemented as

xandr | grep -oP '\b\d+x\d+(?!(?:\+\d+\+)?\d)'

See the online demo and the regex demo.


  • -oP - output matches only and enable PCRE regex engine
  • \b\d+x\d+(?!(?:\+\d+\+)?\d):
    • \b - word boundary
    • \d+ - one or more digits
    • x - an x
    • \d+ - one or more digits
    • (?!(?:\+\d+\+)?\d) - a negative lookahead that fails the match if, immediately to the right of the current location, there is
    • (?:\+\d+\+)? - an optional sequence of +, one or more digits and +
    • \d - a digit.

Regex boundary to also exclude special characters

You may use lookahead and lookbehind on both sides to fail the match if there is a non-whitespace character on either side:


RegEx Demo

  • (?<!\S): Fail if previous character is a non-whitespace
  • (?!\S): Fail if next character is a non-whitespace

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