Using regex in bash scripting - regex

Question

I'm trying to use regex in an if statement in a bash script, but I am getting different values.
The script:
#!/bin/bash
a="input2.txt"
paramCheck(){
while read -r line; do
d=( $line )
e=${d[#]:1}
for i in "$e"; do
if [ "$i" == $[st][0-9] ]; then
echo "$i"
fi
done
done < "$a"
}
echo `paramCheck`
The text file:
add $s1 $s2 $s3
sub $t0
sub $t1 $t0
addi $t1 $t0 $s5
The predicted results:
$s1 $s2 $s3 $t0 $t1 $t0 $t1 $t0 $s5
The actual result was: nothing printed out.

Answer 1

You have to use double brackets for regex matching and escape the dollar, as it is a special bash character. Substitute
if [ "$i" == $[st][0-9] ]; then
for
if [[ "$i" = \$[st][0-9] ]]; then

Answer 2

Here's one way you could do this using various standard utilities:
$ cut -d' ' -f2- infile | grep -o '\$[st][[:digit:]]' | paste -sd ' '
$s1 $s2 $s3 $t0 $t1 $t0 $t1 $t0 $s
cut removes the first space separated column
grep finds all matches of the pattern and prints them one per line
paste gets the output on a single line
In pure Bash:
#!/usr/bin/env bash
while read -ra line; do
for word in "${line[#]:1}"; do
[[ $word == \$[st][[:digit:]] ]] && printf '%s ' "$word"
done
done < 'input2.txt'
reads directly into an array with read -a
no intermediate assignment, loop directly over elements of "${line[#]:1}"
use [[ ]] for pattern matching, escape $, use locale-safe [[:digit:]] instead of [0-9]
use printf instead of echo to suppress linebreaks
Notice that this'll add a trailing blank.
A few pointers for your code:
d=( $line ) relies on word splitting and is subject to filename expansion; if you have a word * in $line, it'll expand to all files in the directory.
e=${d[#]:1} assigns the second and later elements of the array to a single string – now we don't have an array any longer. To keep the array, use e=("${d[#]:1}") instead.
for i in "$e" now has $e containing all the elements in a single string, and the quoting suppresses word splitting, so for the first line, this'll put all of $s1 $s2 $s3 into i instead of just $s1. The intent is probably for i in $e, but that's again subject to word splitting and glob expansion; use an array instead.
[ ] doesn't support pattern matching, use [[ ]] instead. $ has to be escaped.
Glob patterns (used here) are not regular expressions. Check the "Patterns" article in the references for a good overview of the differences.
Bash does understand both == and = within [ ], but == isn't portable (as in "POSIX conformant") – it's a good habit to use = instead. Within [[ ]], it's debatable what to use, as [[ ]] isn't portable itself.
echo `cmd` is the same as just cmd.
References:
cut invocation
grep -o manual
paste invocation
Wooledge wiki article about patterns