# Positive lookahead not working

**URL:** <https://discuss.elastic.co/t/positive-lookahead-not-working/258752>\
**Category:** Logstash\
**Created:** [December 15, 2020, 7:29pm UTC](https://discuss.elastic.co/t/positive-lookahead-not-working/258752 "2020-12-15T19:29:24Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Muhammad\_Faisal](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/muhammad_faisal/32/79213_2.png) [@Muhammad\_Faisal](https://discuss.elastic.co/u/Muhammad_Faisal)\
**Post date:** [December 15, 2020, 7:29pm UTC](https://discuss.elastic.co/t/positive-lookahead-not-working/258752/1 "2020-12-15T19:29:24Z")

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hi guys,

i want to return or get output 40:123 , whereas input is 40$:123 ---- want to use lookahead , but it always return in output if i use below...any idea, need to used lookahead $ is must in named capture.

input string  
40$:123

`(?<pattern>\d+(?=\$)\$:\d+)`

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<div class="post-metadata">

**Author:** ![Badger](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/badger/32/25190_2.png) [@Badger](https://discuss.elastic.co/u/Badger)\
**Post date:** [December 15, 2020, 8:40pm UTC](https://discuss.elastic.co/t/positive-lookahead-not-working/258752/2 "2020-12-15T20:40:35Z")

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What filter are you using? Why do you want to use lookahead? If you want to remove the $ then why not just use mutate+gsub?

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<div class="post-metadata">

**Author:** ![Muhammad\_Faisal](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/muhammad_faisal/32/79213_2.png) [@Muhammad\_Faisal](https://discuss.elastic.co/u/Muhammad_Faisal)\
**Post date:** [December 16, 2020, 7:26am UTC](https://discuss.elastic.co/t/positive-lookahead-not-working/258752/3 "2020-12-16T07:26:02Z")

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i have input like this

key$:value  
Key$:value

i need to check if key is followed by $ but i don't want to return it...i know we can use other options why its not working using lookahead ...

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<div class="post-metadata">

**Author:** ![Badger](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/badger/32/25190_2.png) [@Badger](https://discuss.elastic.co/u/Badger)\
**Post date:** [December 16, 2020, 4:10pm UTC](https://discuss.elastic.co/t/positive-lookahead-not-working/258752/4 "2020-12-16T16:10:15Z")

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You have not given any indication of your filter configuration, so it is impossible to help you.

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<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [January 13, 2021, 4:10pm UTC](https://discuss.elastic.co/t/positive-lookahead-not-working/258752/5 "2021-01-13T16:10:40Z")

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