Regex without 'm' or '/'

The example that BorgCopyeditor supplies with his question brings up an interesting point. Why does this work: if($query=~$term) {...} Or, to offer a very plain example, why does this work:

     if ( 'abc' =~ 'bc' ) {
        print "yes\n";
     }
[download]

The rule I learned was (quoting from perlop): "If "/" is the delimiter then the initial m is optional."

Fair enough. The implication is:

     if ( 'abc' =~ m/bc/ ) {  # good
     if ( 'abc' =~  /bc/ ) {  # good
     if ( 'abc' =~ m%bc% ) {  # good (for any non-alphanum)
  but:
     if ( 'abc' =~  'bc' ) {  # BAD! (or so we assume)
[download]

But it is not so. As I said above, both of the examples at the top of this response work. Why?

What I cannot find in the on-line docs (update: see danger's response below) but I know from experimentation and the words of Camel 3, page 144, is that, even without the 'm' or the '/', the righthand side of =~ "still counts as a m// matching operation, but there'll be no place to put any trailing modifiers, and you'll have to handle your own quoting."

So...

     if ( 'abc' =~ 'bc' ) {       # works
     if ( 'abc' =~ $pattern ) {   # works
     if ( 'abc' =~ "$pattern" ) { # works
     if ( 'abc' =~ bc ) {         # works!!
     if ( 'abc' =~ 'bc'g ) {      # Error: bareword 'g'
[download]

The present writer is not responsible for any sideways looks any of these may earn you from your peers.

------------------------------------------------------------
"Perl is a mess and that's good because the
problem space is also a mess." - Larry Wall

Comment on Regex without 'm' or '/' Select or Download Code

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Re: Regex without 'm' or '/' by danger (Priest) on Jul 10, 2002 at 04:05 UTC
What I cannot find in the on-line docs Its an operator thing, not a regex thing --- from perlop under "Binding Operators": `... If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time. This can be less efficient than an explicit search, because the pat- tern must be compiled every time the expression is evalu- ated.` [download] This can sometimes cause problems for newcomers, especially when they use split with a double-quoted string as the split pattern (as seems to happen with undue frequency) and have an escaped metacharacter in the pattern: `$_ = 'this has a \| pipe'; @a = split /\\|/; # good print join(":", @a),"\n"; @a = split "\\|"; # oops print join(":", @a),"\n";` [download] In the second case, the double-quoted string is first evaluated and the resulting string (sans backslash) is then used as the pattern in the regex.	[reply] [d/l] [select]
Re: Re: Regex without 'm' or '/' by hv (Prior) on Feb 24, 2003 at 01:19 UTC
If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time. This can be less efficient than an explicit search, because the pat- tern must be compiled every time the expression is evalu- ated. This perlop text must be a holdover from a while back. At least as far back as 5.005_03, code like `$str1 =~ 'bc'` (with a constant string for the pattern) would be compiled only once. Between 5.6.0 and 5.6.1 an extra check was added, so that even `$str =~ $str2` would not be recompiled as long as `$str2` had not changed. I guess the second statement should simply be deleted from that paragraph. Hugo	[reply] [d/l] [select]
Thanks, all by BorgCopyeditor (Friar) on Jul 10, 2002 at 02:03 UTC
Thanks to Texas Tess, Zaxo, and dvergin for patient explanations of both the obvious and the arcane. Also, next time I have a regex problem, maybe I'll brave the debugger. That was very enlightening. FWIW, the data I'm parsing is in 'betacode', an ASCII transcription scheme for Ancient Greek. It's convenient in some ways, but chock full of what I still have to remind myself are metacharacters. Grrr. BCE --Your punctuation skills are insufficient!	[reply]