REGEX match for `if` in shells such as Dash (default /bin/sh in Ubuntu)

match_regex.sh

# using expr
expr_match() {
    local text="${1}";
    local rexp="${2}";
    # NOTE: expr regex is ancored with '^'
    expr "${text}" : "${rexp}" > /dev/null;
}

# using grep
grep_match() {
    local text="${1}";
    local rexp="${2}";
    echo "${text}" | grep -E -q "${rexp}";
}

match_regex_test.sh

if expr_match "string" "s"; then
    echo "true"
fi;

BUG in GNU's expr:

$ expr "+" : ".*"
expr: erro de sintaxe: argumento inesperado “.*”

$ expr match "+" ".*"
expr: erro de sintaxe: faltam argumentos após “.*”

The problem doesn't occur in Busybox's expr.

$ busybox expr "+" : ".*"
1

$ busybox expr match "+" ".*"
1