regexstuff.md

Regex Studies:

Back Reference to capturing group that matches nothing:

(re?)txt\1

Back Reference to a capturing group that does not participate in match:

(re)?txt\1

Placing the ? inside the () allows a match of nothing.

Searching on: test(,?)\1 line
test line ==> test line

Placing the ? outside the () results in a failure to match.

Searching on: test(,)?\1 line
test line ==> *No Match*

The key concept here is that a back reference to a capture group that did not participate in the match

• ? outside () - results in a failure to match, whereas a backreference to a capture group that matches nothing,

Positive Lookahead

re_1(?=re_2)

re_2 is not matched, rather it is used to see whether a match is possible by "looking ahead" in the line to see if something is present.

Searching on: ^.*?t(?=.*gordon)
first line of the night for jim ==> *No Match*
best, line of the night for gordon ==> best
last fine - always gordon ==> last

Positive Lookbehind

(?=re_1)re2

Similar to Positive Lookahead, Postive look behind ensures that the match is possible by "looking behind" in the line to see if something is present.

cat f2 | grep -P 'Sport'

Basket Sport
Hockey Sport
Soccer Sport
Football Sport

cat f2 | grep -P '(?<=t )Sport'
Basket Sport