What is a "Nested Quantifier" and why is it causing my regex to fail?

Question

I have this regex I built and tested in regex buddy.

"_ [ 0-9]{10}+ {1}+[ 0-9]{10}+ {2}+[ 0-9]{6}+ {2}[ 0-9]{2}"

When I use this in .Net C#

I receive the exception

"parsing \"_ [ 0-9]{10}+ +[ 0-9]{10}+  +[ 0-9]{6}+  [ 0-9]{2}\" - Nested quantifier +."

What does this error mean? Apparently .net doesn't like the expression.

Here is the regex buddy so u can understand my intention with the regex...

_ [ 0-9]{10}+ {1}+[ 0-9]{10}+ {2}+[ 0-9]{6}+ {2}[ 0-9]{2}

Match the characters "_ " literally «_ »
Match a single character present in the list below «[ 0-9]{10}+»
   Exactly 10 times «{10}+»
   The character " " « »
   A character in the range between "0" and "9" «0-9»
Match the character " " literally « {1}+»
   Exactly 1 times «{1}+»
Match a single character present in the list below «[ 0-9]{10}+»
   Exactly 10 times «{10}+»
   The character " " « »
   A character in the range between "0" and "9" «0-9»
Match the character " " literally « {2}+»
   Exactly 2 times «{2}+»
Match a single character present in the list below «[ 0-9]{6}+»
   Exactly 6 times «{6}+»
   The character " " « »
   A character in the range between "0" and "9" «0-9»
Match the character " " literally « {2}»
   Exactly 2 times «{2}»
Match a single character present in the list below «[ 0-9]{2}»
   Exactly 2 times «{2}»
   The character " " « »
   A character in the range between "0" and "9" «0-9»

In short...

What is a Nested quantifier?

score 17 · Accepted Answer · edited Jan 13 '14 at 14:06

17

.NET is complaining about the + after the {n} style quantifier as it doesn't make any sense. {n} means match exactly n of a given group. + means match one or more of a given group. Remove the +'s and it'll compile fine.

"_ [ 0-9]{10} {1}[ 0-9]{10} {2}[ 0-9]{6} {2}[ 0-9]{2}"

edited Jan 13 '14 at 14:06

Sergio

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answered Oct 16 '08 at 20:48

Duncan

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In certain regex flavours {min,max}+ is a possessive quantifier, but .Net doesn't support them. If you're using Regex buddy, you can tell it which flavour of regex you're working with by right clicking the composition pane and selecting your 'Flavor' from the drop down list. – Grokodile Jun 24 '11 at 15:57

score 16 · Answer 2 · edited Apr 12 '20 at 21:59

16

.NET doesn't support the possessive quantifier

{10}+

However, {10} should have exactly the same effect. The + avoids backtracking and trying shorter matches if the longest match fails, but since {10} can only match exactly 10 characters to start with this doesn't achieve much.

"_ [ 0-9]{10} [ 0-9]{10} {2}[ 0-9]{6} {2}[ 0-9]{2}"

should be fine. I've also dropped the "{1}+" bit .Since it matches exactly once, "A{1}+" is equivalent to just "A".

EDIT As Porges says, if you do need possessive quantifiers in .NET, then atomic groups give the same functionality with (?>[0-9]*) being equivalent to [0-9]*+.

edited Apr 12 '20 at 21:59

silkfire

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answered Oct 16 '08 at 20:56

stevemegson

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Cool. Didn't realise some regex engines provided this option. – Duncan Oct 16 '08 at 21:16
I think you should be able to emulate possessive quantifiers by using atomic groups i.e. instead of `[0-9]{10}+`, use `(?>[0-9]{10})`. – porges May 17 '11 at 20:37

score 2 · Answer 3 · answered Oct 16 '08 at 20:53

2

They're right. This version of your regex doesn't fail:

(_ [ 0-9]{10})+(\s{1})+([ 0-9]{10})+(\s{2})+([ 0-9]{6})+\s{2}[ 0-9]{2}

Notice the use of parens to create groups that then can repeat one or more times. Also, you should be more specific and use \s instead of a space, as pattern whitespace may or may not have significance.

BTW, this regex doesn't look all that useful. You might want to ask another question along the lines of "How do I use regex to match this pattern?"

answered Oct 16 '08 at 20:53

Well this is just a snippet the full regex is this... _ [0-9]{10} {1}[ 0-9]{10} {2}[ 0-9]{6} {2}[ 0-9]{2}|_ [ 0-9]{10} {1}[0-9]{10} {2}[ 0-9]{6} {2}[ 0-9]{2}|_ [ 0-9]{10} {1}[ 0-9]{10} {2}[ 0-9]{6} {2}[0-9]{2} Its returning fields as long as one isn't blank...and I like the \s idea. Thanks – ctrlShiftBryan Oct 16 '08 at 20:57
I'm almost positive that regex could be shrunk down bigtime. Seriously, ask a question about how to do it and provide some sample data. – Oct 16 '08 at 21:01

score 2 · Answer 4 · answered Nov 09 '08 at 15:06

If you select the .NET flavor in the toolbar at the top in RegexBuddy, RegexBuddy will indicate that .NET does not support possessive quantifiers such as {10}+.

Since {10} allows only for one specific number of repetitions, making it lazy or possessive is pointless, even if it is syntactically valid in the regex flavors that support lazy and/or possessive quantifiers. Removing the + signs from your regex will make it work fine with .NET.

In other situations, double-click on the error about the possessive quantifier in the Create tab in RegexBuddy. RegexBuddy will then replace the possessive quantifier with a functionally equivalent atomic group.

If you generate a source code snippet for a .NET language on the Use tab in RegexBuddy, RegexBuddy will automatically replace possessive quantifiers in the regex in the source code snippet.

What is a "Nested Quantifier" and why is it causing my regex to fail?

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