Regex capture groups is a mechanism in regular expressions to define parts of the pattern as their own entity with a unique identifier. This allows to refer to any capture group on the match and execute actions on it.
Capture groups syntax
A capture group is simply defined on a regex pattern with parenthesis
Hello (world)
Each capture group is defined by an integer ID number and the main regex pattern always has ID 0. The previous example results in the following capture groups:
- 0: Hello world
- 1: world
Capture groups can be nested. The following example results in 4 groups:
Hello (b(eau)tiful) (world)
- 0: Hello beautiful world
- 1: beautiful
- 2: eau
- 3: world
Example with capture groups
Use of regex capture groups to convert single-line print statements in Python 2 to new function format print() in Python 3:
s/print\s([^(\s].*)/print(\1)/g
Match pattern:
print: match string literally\s: match a white space character(: start capture group[^(\s]: match a single character that is not(or a white space.*: match any character between zero and unlimited times
): end capture group
Substitution expression takes the first capture group (i.e. everything inside the print statement) and outputs it in-between the parentheses of print().
Note
On
sedit is necessary to either escape the parenthesis of the capture groups or execute it with extended regular expressions enabled with the-roption.
