add pretty printing for regex patterns

src/pycalver/regexfmt.py

@@ -0,0 +1,68 @@
+import re
+import textwrap
+
+from . import pysix
+from . import patterns
+
+
+def format_regex(regex: str) -> str:
+    r"""Format a regex pattern suitible for flags=re.VERBOSE.
+
+    >>> regex = r"\[CalVer v(?P<year_y>[1-9][0-9]{3})(?P<month>(?:1[0-2]|0[1-9]))"
+    >>> print(format_regex(regex))
+    \[CalVer[ ]v
+    (?P<year_y>[1-9][0-9]{3})
+    (?P<month>
+        (?:1[0-2]|0[1-9])
+    )
+    """
+    # provoke error for invalid regex
+    re.compile(regex)
+
+    tmp_regex = regex.replace(" ", r"[ ]")
+    tmp_regex, _ = re.subn(r"([^\\])?\)(\?)?", "\\1)\\2\n", tmp_regex)
+    tmp_regex, _ = re.subn(r"([^\\])\("      , "\\1\n("   , tmp_regex)
+    tmp_regex, _ = re.subn(r"^\)\)"          , ")\n)"     , tmp_regex, flags=re.MULTILINE)
+    lines          = tmp_regex.splitlines()
+    indented_lines = []
+    level          = 0
+    for line in lines:
+        if line.strip():
+            increment = line.count("(") - line.count(")")
+            if increment >= 0:
+                line = "    " * level + line
+                level += increment
+            else:
+                level += increment
+                line = "    " * level + line
+            indented_lines.append(line)
+
+    formatted_regex = "\n".join(indented_lines)
+
+    # provoke error if there is a bug in the formatting code
+    re.compile(formatted_regex)
+    return formatted_regex
+
+
+def pyexpr_regex(regex: str) -> str:
+    try:
+        formatted_regex = format_regex(regex)
+        formatted_regex = textwrap.indent(formatted_regex.rstrip(), "    ")
+        return 're.compile(r"""\n' + formatted_regex + '\n""", flags=re.VERBOSE)'
+    except re.error:
+        return f"re.compile({repr(regex)})"
+
+
+def regex101_url(pattern: patterns.Pattern) -> str:
+    try:
+        regex_text = format_regex(pattern.regexp.pattern)
+    except re.error:
+        regex_text = pattern.regexp.pattern
+
+    return "".join(
+        (
+            "https://regex101.com/",
+            "?flavor=python",
+            "&flags=gmx" "&regex=" + pysix.quote(regex_text),
+        )
+    )