-set_regex¶

Introduction¶

One common feature of many Unix/Linux commands is the support of regular expressions (regex). Several of wgrib2's options support regex (-if, -not_if, -match, -not, -egrep, -egrep_v) which make the various string comparisons much more versatile (compare egrep vs windows find command). The -set_regex option changes the flavor of regex evaluation.

NOTE: the -set_regex option MUST preceed the -match/-not/-if/-not_if/-egrep/-egrep_v options. All these options are "setup" options and the processing the the -set_regex must occur before the processing of the -match and other regex options.

Grib files can have hundreds of records and most people only want a few of the records. Rather than processing everything, you can select the specific records to process by the -match, -not, -if and -not if options. These options take an extended POSIX regular expression (regex) as their sole argument. These options check the regex with the "match inventory" (see -match_inv). Some examples are

  wgrib2 input.grb -match ':UGRD:200 mb:' -grib u.grb
  wgrib2 input.grb -match ':(UGRD|VGRD|TMP):200 mb:' -grib uvt.grb

Now regex are powerful but can produce some surprises. For example, you want the 19^th (positive perturbation) ensemble member which is denoted by 'ENS=+19' in the match inventory. You try,

  wgrib2 input.grb -match ':ENS=+19:' -grib e19.grb

Surprise, the above line does not work. The plus sign is a regex metacharacter indicating that the previous character would be matched 1 or more times. Consequently the plus sign wouldn't be matched. To get the above match to work, you can quote the plus sign with a backslash.

  wgrib2 input.grb -match ':ENS=\+19:' -grib e19.grb

Alternatively you could change the regex match into fixed-string mode. In fixed-string mode, the regex metacharacters are considered to be ordinary characters.

  wgrib2 -set_regex 1 input.grb -match ':ENS=+19:' -grib e19.grb

Most of the regex options have a fs (fixed string) version, such as -match and -match_fs. It is better to used the fixed-string versions are equivalent to the regex mode set to "fixed string". The fixed string versions were added because regex library may be unavaible on non-POSIX systems.

  wgrib2 input.grb -match_fs ':ENS=+19:' -grib e19.grb

The third mode is the metacharacters have to be quoted. Here is an example that gets the 19^th, 20^th and 21th ensemble members. This mode was added because it is easier to remember to quote the '(|)' metacharacters than to quote the ordinary characters correspond to metacharacters.

  wgrib2 -set_regex 2 input.grb -match ':ENS=+\(19\|20\|21\):' -grib e19_20_21.grb

Usage¶

-set_regex X              X=0, 1, 2
                            0 = extended regular expressions
                            1 = fixed string mode, no metacharacters
                            2 = metacharacters need to be quoted

Example¶

$ wgrib2 gep.grb -match ':UGRD:200 mb:' -match ':ENS=+19:'
        (no output)
$ wgrib2 gep.grb -set\_regex 1 -match ':UGRD:200 mb:' -match ':ENS=+19:'
4.1:86046:d=2009060500:UGRD:200 mb:180 hour fcst:ENS=+19