Group, Greedy and Lazy in Regular Expression

(…) means a group, group can have a name, form is: (?’groupname’…) or (?<groupname>…)

each group has a GroupID (from 1 to infinite), group 0 is the whole matched string

If you want to reference group, you may use /k<groupname> or /GroupNumber

For example:

Input: “dream is a dream”

Pattern: “(dream) is a /1”

Result: matched

The first “dream” is group 1, group 0 is “dream is a dream”

Input: “dream is a dream”

Pattern: “(?<drm>dream) is a /k<drm>”

Result: matched

Group “drm” is dream, /k<drm> reference group “drm”

“Greedy” means a pattern will match a string as long as possible, it’s by default, for example:

Input: dreamdream

Pattern: (dream)*

Result: matched

Matched value: dreamdream

We can set an lazy pattern:

Input: dreamdream

Pattern: (dream)*?

Result: matched

Matched value: Empty

Because *? means as less as possible, 0 is the minimum, so it’s empty

Input: dreamdream

Pattern: (dream)+?

Result: matched

Matched value: dream

Because +? means as less as possible, 1 is the minimum, so it’s “dream”, if you use the function “Matches”, you will get 2 Match object, they both are “dream”.