sel_matches_regex
Title
sel_matches_regex - Get variables that match a regular expression.
Syntax
sel_matches_regex regex_string, [varlist(varlist) negate]
| options | Description |
|---|---|
| negate | Returns variables not matched by regex |
| varlist(varlist) | Sets the scope of the regex search |
Description
By default, Stata allows users to specify a list of variables through a range (e.g., var1 - var5) or a (glob) pattern (e.g., var*). See more here.
However, there is no straight-forward way to specify a list of variables that match a regular expression, a pattern specification that is typically more precise than either of the foregoing Stata options. The sel_matches_regex command fills that gap in functionality.
In particular, this function aims to meet a few needs:
- Select variables more precisely
- Identify no variables fail to follow a pattern
See examples below.
Options
negate inverts the regex selection. Rather than return matching variables, this option returns variables that do not match.
varlist(varlist) restricts the scope of the regex search to the user-provided variable list. By default, sel_matches_regex searches for matches in all variables in memory. With varlist, the scope of the search can be narrowed. This narrower varlist could come, for example, from other commands in selector.
Examples
Example 1: Select variables more precisely
* create sets of variables
gen housing_unit = .
gen s01q01_quantity = .
gen s01q01_unit = .
gen s01q02_quantity = .
gen s01q02_unit = .
gen s01q03_quantity = .
gen s01q03_unit = .
gen s01q04_quantity = .
gen s01q04_unit = .
* select variables that end in _unit
sel_matches_regex "_unit$"
* select variables that end in _unit for questions 02 and 03
sel_matches_regex "0[23]_unit$"Example 2: Identify variables that do not follow a pattern
* create a set of variables that mostly follow a pattern
* importantly, some don't
gen s01q01 = .
gen s01q02 = .
gen s01_q03 = .
gen s01q04 = .
gen S01q04 = .
gen s01q05a = .
gen s01q05_unit = .
* identify variables that do NOT follow the pattern
sel_matches_regex "s01q0[0-9][a-z]*$", negate
* assert that there are no variables fail to follow the pattern
* preventing variable naming problems, say, in disseminated data
local pattern_for_data "s01q0[0-9][a-z]*$"
qui: sel_matches_regex "`pattern_for_data'", negate
local not_follow = r(varlist)
local n_not_follow : list sizeof not_follow
capture assert n_not_follow == 0
if _rc != 1 {
di as error "Some variables do not follow the desired pattern (`pattern_for_data')"
di as text "`not_follow'"
}Feedback, Bug Reports, and Contributions
Read more about these commands on this repo where this package is developed. Please provide any feedback by opening an issue. PRs with suggestions for improvements are also greatly appreciated.