RE: Why doesn't this work: matching capturing

Paul, thank you very much for your helpful reply. To answer your question, I am certain that the first two matches worked because I produced the output I showed with:
[kevinz@www UScensus]$ perl -ne 'print if /^.{8}150.{18}24510.{21}(.{6})(.)/;' mdgeo.uf1 |head -1
uSF1 MD15000000 009214935522451020 9 0101001 88722397N07209999900 116759 0Block Group 1 S 1158 662+39283007-076574503
[kevinz@www UScensus]$

Sorry if I left out this information and wasted anyone's time.

A person responded to me privately and suggested this modification, which seems to work fine:
[kevinz@www UScensus]$ perl -ne 'print "Tract $1 BLKGRP $2\n" if /^.{8}150.{18}24510.{21}(.{6})(.)/;' mdgeo.uf1 |head -3
Tract 010100 BLKGRP 1
Tract 010100 BLKGRP 2
Tract 010100 BLKGRP 3
[kevinz@www UScensus]$

I've been bitten by this bug before and can never remember the solution to when variables are assigned values. I thought assigning them in a previous command would have worked, but I must have overlooked something.

Thank you, again, for your help.

-Kevin

-----Original Message-----
From: Paul Lalli [mailto:mritty@xxxxxxxxx]
Sent: Tuesday, February 26, 2008 3:41 PM
To: beginners@xxxxxxxx
Subject: Re: Why doesn't this work: matching capturing

On Feb 26, 1:19 pm, kzemb...@xxxxxxxxxx (Kevin Zembower) wrote:
I have a data file that looks like this:
uSF1  MD15000000  009214935522451020                9  0101001
88722397N07209999900
116759             0Block Group 1
S      1158      662+39283007-076574503

uSF1  MD15000000  009215035522451020                9  0101002
88722397N07209999900
109338             0Block Group 2
S       842      547+39280857-076573636

uSF1  MD15000000  009215135522451020                9  0101003
88722397N07209999900
182248        135142Block Group 3
S       920      442+39279557-076574311

This is actually three lines that all start with 'uSF1'. This is the
Summary File from the US 2000 Census. I want to print all the census
tracts and blockgroup numbers for FIPS state code = "24" (Maryland) and
FIPS county code "510" (Baltimore City) for summary level '150'. These
are all fixed-length records. I tried:
[kevinz@www UScensus]$ perl -ne '($tract, $bg) =
/^.{8}150.{18}24510.{21}(.{6})(.)/; print "Tract $tract BLKGRP $bg\n";'
mdgeo.uf1 |head
Tract  BLKGRP
Tract  BLKGRP
Tract  BLKGRP
<snip>

I thought that this would:
   skip 8 characters and match '150'
   skip 19 more characters and match '24' and '510'
   skip 21 more characters and capture the next 6 in $tract
   capture the next character in $bg
   and print them.

The first two matches work, but nothing is captured. Any ideas what I'm
doing wrong?

On what do you base your assumption that "the first two matches
work"? Nothing in your code or output indicates that, as you are
never checking the return value of the pattern match.

FWIW, your code did work for me when I copy and pasted your sample
text, and joined the lines as they should have been. Therefore, I
think it's pretty likely that your datafile does not contain what you
think it does. I think it's more likely that the one line you think
you have that starts with uSF is actually broken up into a few lines.

Try some debugging prints of $_ to see what you actually have, like:
print "Line $.: <<$_>>";

Try checking the return value of your regexp:
/^.{8}150.{18}24510.{21}(.{6})(.)/ and print "Tract $1 BLKGRP $2\n";

Try enabling warnings to see of your two variables are undefined
(which they would be if the pattern didn't match) or just empty
strings (which they would be if the pattern matched but nothing was
captured - this, of course, isn't possible, since a six-character
match can't possibly be the empty string).

Paul Lalli


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