regexp.md

Misc

• Often during parsing happens that both left and right sides of the regexp match. In this case left side is being preferred unless there's a lazyfying ?, which makes the right one being preferred.
• Look-ahead/look-behind: (perl syntax) is a groups that starts with ?, then another quantifier goes. Example: (?=…) and (?<=…): positive look-ahead and look-behind, (?!…) and (?<!): negative look-ahead and look-behind.
  • "Negative look-ahead example": matching a this in lines that do not contain // before this: grep -P '^(?!.*//)^.*?\bthis\b' -- foo.c. Here we first filter the line through the ^(?!.*//) and then through the ^.*?\bthis\b. Notice the ^ on the right side. It isn't really needed here, but I added it just to show that the left and the right sides are being matched separately on the whole line.
  • These groups are "non-capturing".