3.6 Basic vs Extended Regular Expressions

Basic regular expressions differ from extended regular expressions in the following ways:

• The characters ‘?’, ‘+’, ‘{’, ‘|’, ‘(’, and ‘)’ lose their special meaning; instead use the backslashed versions ‘\?’, ‘\+’, ‘\{’, ‘\|’, ‘\(’, and ‘\)’. Also, a backslash is needed before an interval expression’s closing ‘}’.
• An unmatched ‘\)’ is invalid.
• If an unescaped ‘^’ appears neither first, nor directly after ‘\(’ or ‘\|’, it is treated like an ordinary character and is not an anchor.
• If an unescaped ‘$’ appears neither last, nor directly before ‘\|’ or ‘\)’, it is treated like an ordinary character and is not an anchor.
• If an unescaped ‘*’ appears first, or appears directly after ‘\(’ or ‘\|’ or anchoring ‘^’, it is treated like an ordinary character and is not a repetition operator.

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