There is a regular expression that I used often in Mathematica 8, made to locate matching curly braces:


However, it is no longer working in Mathematica 10.4. It returns an error

RegularExpression::msg84: Group name must start with a non-digit [[etc.]] 

Any clue what is going on?

  • 2
    $\begingroup$ Would you give us an example of how you use the RE you show fails. I do not find "to locate matching curly braces" sufficient to be able to reproduce the problem $\endgroup$
    – m_goldberg
    May 28, 2017 at 23:03
  • $\begingroup$ I don't find that RegularExpression["(?P<0>{([^{}]|(?P>0))*})"] returns an error in my copy of 10.4 (Mac OS 10.10) $\endgroup$
    – QuantumDot
    May 28, 2017 at 23:46
  • $\begingroup$ Replace the 0 in name with something with a non-digit initial... $\endgroup$
    – ciao
    May 28, 2017 at 23:47

1 Answer 1


Fix the group name. Beats me why the change - first works in 9.X Win., but not on 10.3 Win...

StringPosition["{tes,{1,2,3},t}", RegularExpression["(?P<0>{([^{}]|(?P>0))*})"]]

RegularExpression::msg84: Group name must start with a non-digit in RegularExpression[(?P<0>{([^{}]|(?P>0))*})]. >>

StringPosition["{tes,{1,2,3},t}", RegularExpression["(?P<grp0>{([^{}]|(?P>grp0))*})"]]

{{1, 15}, {6, 12}}

  • 5
    $\begingroup$ PCRE used to allow names starting with digits. But version 8.34 of PCRE disallowed them -- to match the same change which was made in Perl. It appears that Mathematica 10.1 was the first release to use PCRE 8.34 or later. $\endgroup$
    – WReach
    May 29, 2017 at 3:07
  • $\begingroup$ @WReach - Thought as much, but did not want to shoot the mouth without knowing it to be true. Thanks for confirmation. $\endgroup$
    – ciao
    May 29, 2017 at 7:36

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