expr: Handle `\{` literally at the start of an expression or subexpression

[frendsick]

Normally, \{ begins a range quantifier like {n,m}. However, at the start of the regex or its subexpression (after \( or \|), there is no preceding item to which the quantifier can be applied. The expr command should handle \{ literally at the start of the regex or its subexpression.

Examples

• expr '{abc}' : '\{abc\}'
  • Expected: "5"
  • Got: "expr: Invalid content of {}"
• expr 'a{bc}' : 'a\(\{bc\}\)'
  • Expected: "{bc}"
  • Got: "expr: Invalid content of {}"
• expr '{b}' : 'a\|\{b\}'
  • Expected: "3"
  • Got: "expr: Invalid content of {}"
• expr '{' : 'a\|\{'
  • Expected: "1"
  • Got: expr: Unmatched \{

[frendsick]

I can take care of this issue.