What does this regex expression in Qt mean? I can't understand the meaning behind the ?=.

QRegularExpression functionPattern("\\b[A-Za-z_][A-Za-z0-9_]*(?=\\()");

P.S.: This is a regex expression about parsing the C language function name.

CodePudding user response：

First, remember to unescape the double backward slashes \\ into a single backslash \, obtaining the actual regex \b[A-Za-z_][A-Za-z0-9_]*(?=\().

Feeding the above regex into

CodePudding user response：

As Nole pointed out, you should unescape the double backward slashes into a single backslash. A single backslash followed by certain characters has special meaning. E.g., \b means the boundary of a word and it doesn't capture anything. For example, \bword\b matches word, something, word, something else, but not password. (?=…) is a positive lookahead and it is a non-capturing group, i.e., it doesn't capture anything. It means that there should be … in that position. In our case, (?=\() means there should be ( in the position. Note that the single backslash before ( used to mean the literal ( and not its meaning in RegEx context, i.e., grouping.

The whole pattern means a word (and not part of a word, since we used \b at the beginning) that Starts with a letter or an underscore ([A-Za-z_]) that "may" followed by a letter, an underscore or a number ([A-Za-z0-9_]*; "may" refers to the *). And it should be followed by (.

Note again that this pattern captures whole the word up until the ( and not ( itself.