Search for pp_substr in pp.c.

It's not a good idea to want to modify substr on that level. You can use subs (I think) and override substr with your own custom function. That makes it still a bad idea, but at least you can write your replacement in Perl that way.

What problem are you trying to solve that involves replacing substr?

Update: subs seems to be the wrong module, but I'm sure I remember a module that allowed one to replace built-ins. Maybe I'm just thinking of CORE::GLOBAL:: though:

BEGIN{
    *CORE::GLOBAL::substr=sub($$$) { die };
}
print substr 'foo',2,3;
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I want to understand how it is working and then write my own subroutine for my script

Suppose i have string that contains tags like "<bold>" it should not count this tag. If string has something like "<194>" I want substr to consider it as one character.

Thank you very much

Then write your own implementation of substr instead of replacing it. Most likely you'll need to write a parser for whatever language you're parsing. It shouldn't be hard, and "replace substr" clutters your problem with many hard (and weird) problems that are unrelated to the task.

Write an ordinary Perl program that filters the text, then measures its length using the built-in substr function.

You're trying to solve an everyday text processing problem in a very peculiar and unconventional way. Writing a custom substr function to measure a particular kind of "string" is like inventing a bathroom scale that knows when your hair is wet and you haven't removed your shoes, yet accurately measures your bone-dry, barefoot weight.

I'm curious: Do you want to measure the lengths of the strings in bytes, in encoded characters (Unicode code points), or in real-world characters (Unicode extended grapheme clusters)?

CLARIFICATION: I admit I sort of conflated substr and length in this post. My excuse is that I was fixated on the words "count" and "one character" in tej's restatement of his Y problem:

Suppose i have string that contains tags like "<bold>" it should not count this tag. If string has something like "<194>" I want substr to consider it as one character.

This is most definitely an XY problem. Sounds like you want to do something like the following, but you haven't really said what you are trying to do.

# Tokenise
my @tokens = $doc =~ /<[^>]*>|./sg;

# Extract text
my @text_tokens = grep /^<|<[0-9]+>/, @tokens;

# Do stuff with @text_tokens
# ...

# Print resulting text
print join('', @text_tokens);
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or maybe

# Tokenise
my @tokens = $doc =~ /<[^>]*>|./sg;

# Extract text
my $text = '';
for (@tokens) {
   if    (/^<([0-9]+)>/) { $text .= chr($1); }
   elsif (/^[^<]/)       { $text .= $1;      }
}

# Do stuff with $text
# ...

# Print resulting text
print join('', $text);
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Or do you need to output back in the original format (e.g. with bold tags in)?

Update: Added second snippet.

perl -ne 'print "$ARGV:$.:$_" if /substr/ .. /^}/' pp.c
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Or click "Read more..." below. :-)

Read more... View the pp_substr definition from pp.c (6 kB)