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Using BASH =~ regex to match multiple strings

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Top Forums Shell Programming and Scripting Using BASH =~ regex to match multiple strings
# 8  
Old 04-28-2014
Quote:
Originally Posted by forrie
So am I to infer from the responses that what I am attempting to do cannot be done in this context within BASH?
No, not at all...

Quote:

FYI, this does not work:

Code:
[[ "$1" =~ "one|two|three" ]]

That is a string match, not a regex match see other comments...
Quote:
basically, I'm looking to generate a positive result if it matches any of those words.

I've seen other examples (in different context) that will do something:

Code:
[[ "$1" =~ (regex) ]]

but with the emphasis that "regex" is very much platform-dependent (ie: depends highly on the regular expressions libs available on that system.
Where did you get that idea? That is not correct.. It does depend on bash version. Old versions of bash cannot process =~, (but a case statement using unix patterns will work)

------------
Quote:
Originally Posted by forrie
On CentOS 6.5, I am unable to get this to work correctly (per above):

Code:
[[ $STRING =~ ^(one|two|three)$ ]] && do-something

The string will actually be a part of a hostname, for example "one-hostname" or something like that. I need it check that there's a match, possibly doing something based on what it matched. But in this case the match word will be at the beginning ^one.
What happens when you execute:

Code:
STRING=one; [[ $STRING =~ ^(one|two|three)$ ]] && echo hello

in a bash shell?
  • How do you execute your script?
Last edited by Scrutinizer; 04-28-2014 at 03:59 PM..
# 9  
Old 04-28-2014
@forrie,
consider the following:

Code:
$ echo $BASH_VERSION
4.1.10(4)-release
p='^(one|two|three)$'
$ for s in one two three "$p"; do
>   [[ $s =~ $p ]] && printf '%s matches %s\n' "$s" "$p"
>   [[ $s =~ "$p" ]] && printf '%s matches "%s"\n' "$s" "$p"
> done
one matches ^(one|two|three)$
two matches ^(one|two|three)$
three matches ^(one|two|three)$
^(one|two|three)$ matches "^(one|two|three)$"

As I said, when you quote the regular expression, it's taken literally.
As far as I know, the =~ operator is bash version specific
(i.e. it's not available in older bash versions).
There are some other gotchas and some platform specific issues, see the BashWiki for more info (see Portability Considerations).
Last edited by radoulov; 04-28-2014 at 04:10 PM..
# 10  
Old 04-28-2014
I got this to work:

Code:
[[ $STRING =~ ^(one|two|three)$ ]] && echo hello

This also matches a hostname like
Code:
one-hostname

I'm running BASH
Code:
GNU bash, version 4.3.11(1)-release (x86_64-apple-darwin13.1.0)

The problem was user-related :-) I had our internal strings on the brain (dev, prod, test) and was under-caffeinated.

Thank you all for your helpful pointers.
# 11  
Old 05-01-2014
Quote:
Originally Posted by forrie
I got this to work:

Code:
[[ $STRING =~ ^(one|two|three)$ ]] && echo hello

This also matches a hostname like
Code:
one-hostname

I'm running BASH
Code:
GNU bash, version 4.3.11(1)-release (x86_64-apple-darwin13.1.0)

The problem was user-related :-) I had our internal strings on the brain (dev, prod, test) and was under-caffeinated.

Thank you all for your helpful pointers.
If STRING is set to one-hostname, the command:
Code:
[[ $STRING =~ ^(one|two|three)$ ]] && echo hello

should not produce any output. If you're trying to match one, two, or three at the start of the string and accept anything following one of those three strings, you don't want the $. With the $, the RE matches only if one of those three strings is the entire contents of $STRING.
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