Regex in c++ for maching some patters

Question

I want regex of this.

• add x2, x1, x0 is a valid instruction;

I want to implement this. But bit confused, how to, as I am newbie in using Regex. Can anyone share these Regex?

Answer 1

If this is a longer project and will have more requirements later, then definitely a different approach would be better.

The standard approach to solve such a problem ist to define a grammar and then created a lexer and a parser. The tools lex/yacc or flex/bison can be used for that. Or, a simple shift/reduce parser can also be hand crafted.

The language that you sketched with the given grammar, may be indeed specified with a Chomsky class 3 grammar, and can hence be produced gy a regular grammar. And, with that, parsed with regular expressions.

The specification is a little bit unclear as to what a register is and if there are more keyowrds. Especially ecall is unclear.

But how to build such a regex?

You will define small tokens and concatenate them. And different paths can be implemented with the or operator |.

Let's give sume example.

• a register may be matched with a\d+. So, an "a" followed by ome digits. If it is not only "a", but other letters as well, you could use [a-z]\d+
• op codes with the same number of parameters can be listed up with a simple or |. like in add|sub
• For spaces there are many solutions. you may use \s+ or [ ]+or whatever spaces you need.
• To build one rule, you can concatenate what you learned so far
• Having different parts needs an or | for the complete path
• If you want to get back the matched groups, you must enclose the needed stuff in brackets

And with that, one of many many possible solutions can be:

^[ ]*((add|sub)[ ]+(a\d+)[ ]*,[ ]*(a\d+)[ ]*,[ ]*(a\d+)|(ecall))[ ]*$

See example in: regex101