Regular Expressions ungreedy in bash

Question

Choose which of the following strings match regular expression

(1 U 22)*2*

a. 22112222112211
b. 11112
c. The empty string.
d. 12121
e. 1121111222

I did a few search, U means " Ungreedy. Makes the quantifiers *+?{} consume only those characters absolutely necessary to form a match, leaving the remaining ones available for the next part of the pattern. When the "U" option is not in effect, an individual quantifier can be made non-greedy by following it with a question mark. Conversely, when "U" is in effect, the question mark makes an individual quantifier greedy. " https://www.cheatography.com/davechild/cheat-sheets/regular-expressions/

but I totally don't understand it, what does greedy regular expression and ungreedy regular expression mean? and can you show the example that I listed above?

use https://regex101.com/ to visualize what is happening in debugger mode — rock321987, Apr 27 '16 at 07:36
Greedy matches the longest matching string. Ungreedy matches the shortest. — 123, Apr 27 '16 at 07:39
@rock321987 no that's not duplicate question, all I want to know is how to use the option `U` here with regular expression and understand what is ungreedy — linux, Apr 27 '16 at 07:40
@123 could you provide some simple example to demonstrate that? Sorry I am very new to linux.... — linux, Apr 27 '16 at 07:41
*but I totally don't understand it, what does greedy regular expression and ungreedy regular expression mean?* makes this question a dupe. If you want to know how to use lazy matching in Linux, see [this answer](http://stackoverflow.com/a/34311727/3832970). — Wiktor Stribiżew, Apr 27 '16 at 08:11

score -1 · Answer 1 · answered Apr 27 '16 at 08:01

-1

Greedy means that it will try to find the longest matching string.

For the following string:

{ this} is a { test} }

Example of a Greedy regex

\{.*\}

This regex would match the whole following text:

{ this} is a { test} }

Non Greedy

\{.*\}

would match only { this}

answered Apr 27 '16 at 08:01

Felipe Sulser

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Where's `?`: `\{.*?\}`? – heemayl Apr 27 '16 at 08:02

Regular Expressions ungreedy in bash

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