Return a regex match in a Bash script, instead of replacing it

Question

I just want to match some text in a Bash script. I\'ve tried using sed but I can\'t seem to make it just output the match instead of replacing it with something.

Answer 1

I Know this is an old topic but I came her along same searches and found another great possibility apply a regex on a String/Variable using grep:

# Simple
$(echo "TestT100String" | grep -Po "[0-9]{3}")
# More complex using lookaround
$(echo "TestT100String" | grep -Po "(?i)TestT\K[0-9]{3}(?=String)")

With using lookaround capabilities search expressions can be extended for better matching. Where (?i) indicates the Pattern before the searched Pattern (lookahead), \K indicates the actual search pattern and (?=) contains the pattern after the search (lookbehind).

https://www.regular-expressions.info/lookaround.html

The given example matches the same as the PCRE regex TestT([0-9]{3})String