# How to get prefix of a string
**URL:** https://discuss.elastic.co/t/how-to-get-prefix-of-a-string/139915
**Category:** Logstash
**Created:** [July 13, 2018, 9:38am UTC](https://discuss.elastic.co/t/how-to-get-prefix-of-a-string/139915 "2018-07-13T09:38:11Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@vschakka](https://discuss.elastic.co/u/vschakka)
#### Post date: [July 13, 2018, 9:38am UTC](https://discuss.elastic.co/t/how-to-get-prefix-of-a-string/139915/1 "2018-07-13T09:38:11Z")
Hi,
I have a field with value "sv4axmaic04". I need to extract value before "axmaic". But substring "axmaic" is same/constant across all the field values. So I just need to extract value "sv4" from the given example.
Can I use dissect as below: Or any other better way
dissect {
mapping =\> {
"message" =\> "%{field1}axmaic%{field2}"
}
}
But i don't want field2 getting created here.
---
### Author:  [@Badger](https://discuss.elastic.co/u/Badger)
#### Post date: [July 13, 2018, 2:46pm UTC](https://discuss.elastic.co/t/how-to-get-prefix-of-a-string/139915/2 "2018-07-13T14:46:59Z")
If you do not want field2 created then do not give it a name, just use %{}.
Another option would be to use a regexp to match the first three characters of the field
```
grok { match => { "message" => "^(?...)" } }
```
---
### Author:  [@vschakka](https://discuss.elastic.co/u/vschakka)
#### Post date: [July 16, 2018, 9:02am UTC](https://discuss.elastic.co/t/how-to-get-prefix-of-a-string/139915/3 "2018-07-16T09:02:14Z")
Thanks.
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [August 13, 2018, 9:02am UTC](https://discuss.elastic.co/t/how-to-get-prefix-of-a-string/139915/4 "2018-08-13T09:02:14Z")
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