regex - Sliding window pattern match in perl or matlab regular expressions -

i trying use either perl or matlab parse few numbers out of single line of text. text line is:

t10_t20_t30_t40_

now in matlab, used following script

str = 't10_t20_t30_t40_'; = regexp(str,'t(\d+)_t(\d+)','match') 

and returns

a =   't10_t20'    't30_t40' 

what want return 't20_t30', since match. why doesn't regexp scan it?

i turned perl, , wrote following in perl:

#!/usr/bin/perl -w $str = "t10_t20_t30_t40_"; while($str =~ /(t\d+_t\d+)/g) {     print "$1\n"; } 

and result same matlab

t10_t20 t30_t40 

but wanted "t20_t30" in results.

can tell me how accomplish that? thanks!

[update solution]: colleagues, identified solution using so-called "look-around assertion" afforded perl.

#!/usr/bin/perl -w $str = "t10_t20_t30_t40_"; while($str =~ m/(?=(t\d+_t\d+))/g) {print "$1\n";} 

the key use "zero width look-ahead assertion" in perl. when perl (and other similar packages) uses regexp scan string, not re-scan scanned in last match. in above example, t20_t30 never show in results. capture that, need use zero-width lookahead search scan string, producing matches not exclude substrings subsequent searches (see working code above). search start zero-th position , increment 1 many times possible if "global" modifier appended search (i.e. m//g), making "greedy" search.

this explained in more detail in this blog post.

the expression (?=t\d+_t\d+) matches 0-width string followed t\d+_t\d+, , creates actual "sliding window". returns t\d+_t\d+ patterns in $str without exclusion since every position in $str 0-width string. additional parenthesis captures pattern while doing sliding matching (?=(t\d+_t\d+)) , returns desired sliding window outcome.

using perl:

#!/usr/bin/perl use data::dumper; use modern::perl;  $re = qr/(?=(t\d+_t\d+))/;  @l = 't10_t20_t30_t40' =~  /$re/g; dumper(\@l); 

output:

$var1 = [           't10_t20',           't20_t30',           't30_t40'         ];