CODEWITHSUNDEEP
regexpcreregex-lookarounds
MusikAnimal
MusikAnimal

Reputation: 2416

In regex, match instances of a number but only if not accompanied by any other number

It was terribly hard for me to make a fitting title for this question. It is much more easily explained by example:

test test test 2014 test test // 2014 (truthy)
2014testtest test2014testtest // [2014, 2014] (truthy)
test20141234testtest 2014test // => nothing (falsey)
test 1234 test 2014 test 2014 // => nothing (falsey)

So I want to know if the number 2014 is in the string, not accompanied by another other number except other instances of 2014. Spaces need to be accounted for. As far as whether or not I get an array of matches back, I don't really care. This is truthy/falsey situation.

I'm working with PCRE syntax. Many thanks for the help.

Upvotes: 1

Views: 92

Answers (3)

Unihedron
Unihedron

Reputation: 11041

You can use s and s:

^\D*+\b(\d+)\b\D*(\b\1\b\D*)*+$

Here digit segments are asserted on word boundaries and a backreference ensures the entire string replicates the same digit segments.

Here is a regex demo, however as the test cases are multiline [^\d\n] is used instead. For your actual use case, this is not necessary.

Upvotes: 0

Tim Pietzcker
Tim Pietzcker

Reputation: 336238

If 2014 is the only sequence of valid digits that may occur anywhere in the string (but at least once), then the regex is rather simple:

^(?:\D*2014)+\D*$

Test it live on regex101.com.

Explanation:

^      # Start of the string
(?:    # Start a non-capturing group that matches...
 \D*   # any number of characters except digits
 2014  # followed by "2014".
)+     # Do this any number of times (but at least once).
\D*    # After that, match any remaining non-digits...
$      # until the end of the string.

Upvotes: 4

Gyro Gearloose
Gyro Gearloose

Reputation: 1154

I don't think this can be done by regular expressions in one step. It sure can't be done by a finite automaton, but "regular expressions" as implemented by most languages have capabilities beyond that.

Try this:

substitute all consecutive non-digits by ';'

extract the first sequence of digits and substitute it in the entire string by a dummy ("a")

check if there are any digits left

Upvotes: 0

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